Double bag vacuum infusion process

ABSTRACT

The double bag vacuum infusion process of the present invention provides a low cost method for producing complex composite assemblies without an autoclave. It also enables the production of highly innovative structures. The quality of the composites produced using such an infusion process are comparable to composites made using prepregs, hand layup or fiber placement, and autoclave curing. Double bagging provides vacuum integrity, controls bag relaxation while flow media controls the flow front to allow high quality aerospace-grade products.

REFERENCE TO RELATED APPLICATION

The present application claims the benefit of U.S. Provisional PatentApplication 60/169,531, filed Dec. 7, 1999.

TECHNICAL FIELD

The present invention relates to methods for making composites and tothe products so made, particularly to liquid molding processes includingvacuum-assisted resin transfer molding (VaRTM) or resin infusion.

BACKGROUND OF THE INVENTION

The marine, automotive, trucking, rail, aerospace, defense, recreation,chemical, infrastructure, and other industries look to compositematerials to take advantage of their unique properties, especially beingcorrosion-free or corrosion-resistant and having a highstrength-to-weight ratio. Composites are also resistant to fatigue andchemical attack. They offer high strength and stiffness potential inlightweight components. There is a need, however, to develop compositemanufacturing processes that dramatically reduce their cost ofcomposites, especially large structures, while retaining their highstrength and stiffness.

Resin-impregnated fibrous materials (prepregs) generally are placed on aforming mandrel (“laid up”) by hand or machine using tape, fiber tows,or cloth. Composites also have been made using filament winding.Debulking is required between plies in a laminate to remove air beforethe layups are vacuum bagged (i.e., enclosed in an inert atmosphereunder vacuum to withdraw emitted volatiles released during cure of theresin) and consolidated in autoclaves or presses to achieve high fibervolume components. The prepreg materials typically are expensive(especially those using high modulus carbon fiber). The raw prepregmaterials have limited shelf lives, because the resins react at slowrates (“advance”) at ambient temperature. Advance of the resin adverselyeffects the properties of the resulting composite. Working with prepregalso often results in considerable material waste.

The autoclaves and presses used for consolidation are expensive capitalitems that further increase the final, manufactured cost of thecomposite. Processing has to be centralized and performed in batcheswhere the autoclave or press is installed. Loading and unloading theautoclave (a high temperature, pressurized oven) usually become the ratelimiting steps. The location of the autoclave dictates where thecomposites will be made, so the flexibility of the process is impaired.A dedicated workforce and facility are required, centered around theautoclave.

As mentioned, the prepregs have a limited shelf life. In someformulations, the resin is carried onto the fiber as a lacquer orvarnish containing the monomer reactants that will produce the desiredpolymer in the composite (i.e., prepregs of the PMR-type). In otherformulations, the resin is a relatively low molecular weight polymerthat crosslinks during cure to form the desired polymer. The resin isheld and used in its incomplete state so that it remains a liquid, andcan be impregnated onto the fiber or fabric. Reaction of the monomerreactants or crosslinking of the polymer (i.e., its advancing) prior tothe intended cure cycle adversely impacts the quality of the composite.

Liquid molding techniques such as transfer molding, resin film infusion,resin transfer molding, and structural reaction injection molding (SRIM)typically require expensive matched metal dies and high tonnage pressesor autoclaves. Parts produced with these processes are generally limitedin size and geometry. The conventional liquid molding resins do notprovide the necessary properties for many applications for thecomposites.

Open mold wet layup processing can make large composites using a liquidmolding process with minimal capital equipment, single sided tooling,and often can use lower cost materials than prepreg. The quality anduniformity of the product, however, varies considerably and the bestcomposites are still relatively low quality. The process also tends tobe unfriendly and presents hazards to workers because of their risk ofexposure to the solvents and resins.

Our double bag vacuum infusion (DBVI) process solves a number ofproblems encountered with previously developed, nonautoclave, singlebag, liquid molding techniques, such as those processes described inU.S. Pat. Nos. 4,902,215 (Seemann) and 4,942,013 (Palmer). In theSeemann single bag technique, preferential flow and pressure is inducedin the flow media above the fiber preform. The driving force is apressure differential or head pressure created primarily by drawing downthe pressure inside the bag using a vacuum pump. Atmospheric pressure onthe resin feed pushes resin into the bag through an inlet tube. Resinentering the bag encounters the flow media used to channel the resin tothe underlying fiber preform. Resin flows laterally through the flowmedia over the preform and, subsequently, downwardly into the preform.The preform has the lowest permeability to flow (i.e., the highestresistance to the flow of resin).

Once the liquid media (i.e., ‘resin’) is pulled (i.e., flows) into thepreform, we have observed that the single bag tends to relax behind thewave front (i.e., the foremost portion of the resin that is moving intothe preform within the bag). When the flow media is full or partiallyfull of resin, we believe that the bag slowly relaxes and moves awayfrom the flow media presumably because the flow path of least resistancebecomes a path over the flow media between the flow media and theoverlying bag. Relaxation increases the enclosed volume around thepreform, which becomes filled with resin. The farther away from theleading edge of the wave front, the more the bag tends to relax. We haveobserved that the composite in areas where the bag has relaxed can havelower fiber volume, poor fiber volume control, and lower mechanicalproperties than desired, because excess resin has filled the enlargedvolume. The bag relaxation can produce a change in the intendedthickness of the composite, so that in localized areas where relaxationhas occurred the composite is thicker than intended.

In the ensuing discussion, we will compare the Seemann and Palmerprocesses with our preferred double bag process of the presentinvention.

Our preferred double bag vacuum infusion process circumvents the Seemann(single bag) problems in that the inner and outer vacuum bagsindependently control the resin feed. The double bag provides a cauleffect. The bleeder and breather sections are completely isolated. Withthis approach, the bag is never able to relax behind the wave front andthe resulting composites have higher fiber volumes on average (with moreprecise control) and have uniform thickness with constant thicknesspreforms. Our process eliminates the bag relaxation defects we observedwith the Seemann process.

During relaxation, we observe that resin pools inside the bag. Pressingon the pool, we feel a soft, spongy, loose area different from the feelwhere relaxation is not occurring. The bag stretches and the volumeunder the bag increases. In circumstances of relaxation, we haveobserved that pressurizing the resin feed above atmospheric increasesthe relaxation, so the phenomenon appears to be tied to the pressuredifferential and the driving force for resin flow, as we would expect.Adding a second vacuum bag (separated from the first bag with abreather) makes it harder for the “double bag” to relax. Therefore, wecan use a higher differential pressure to move the resin than might bestbe employed with a single bag. The “double bag” becomes a means toreduce flow over the filled flow media because the vacuum bageffectively is thicker. The “double bag” also provides increased vacuumintegrity because it provides a redundant, second bag to counter anyleaks in the first bag.

In Boeing's “Controlled Atmosphere Pressure Resin Infusion” (CAPRI)process, Jack Woods et al. control the differential pressure by reducingthe pressure below atmospheric in the resin feed tank. In the CAPRIprocess, a vacuum pump evacuates the volume under the vacuum bag while,simultaneously, reducing the pressure over the feed resin. Pressure inthe vacuum bag might be ³¹ 20 inches Hg below atmospheric and ⁻5 inchesHg in the feed pot for a differential pressure to drive resin infusionof 15 inches Hg (˜0.5 atm).

The Palmer process attempted to isolate the bleeder and the breathersections by placing an impervious film between the flow media and thebreather inside the single bag. Unfortunately, this technique did notallow complete isolation. Once the liquid medium reached the vacuum endof the assembly, the flow media and the breather were connected. As aresult, the resin began to wet the breather and to flow back toward theresin source over the membrane because this path had higher permeabilitythan flow downwardly through the preform.

Our preferred ‘double bag’ process allows fiber volume percentage orfraction in the composite to be increased 5-10% higher than we have beenable to achieve with the single bag technologies of Seemann and Palmer.An increased fiber volume is critical to achieve an aerospace gradecomposite that has properties competitive with conventional vacuumbag/autoclave prepreg technologies commonly used in aerospace. Aerospacecomposites have superior ‘specific strengths’ which are achieved byoptimizing (making as high as possible) the fiber volume fraction.Aerospace composites have superior ‘specific strengths’ which areachieved by optimizing (making as high as possible) the fiber volumefraction. Our process achieves a targeted fiber volume within a closetolerance of acceptable fiber volumes by regulating the vacuum of theinner and outer bags during infusion. Using end game thermal infusionstrategies, our process improves preform nesting, fluid drawdown,thermal vacuum debulking, real time mass balance control. Our processhas extremely high vacuum integrity.

In any vacuum impregnation process, vacuum integrity is essential toproduce high quality composites consistently. Leaks in the baggingseals, resin ports, or vacuum ports will permit air to enter into thebag. Air causes the preforms to swell and reduces the fiber volumefraction by increasing the spacing between fibers. Composites made withleaking bags will typically have one of more of the following problems:high void content, surface porosity, low fiber volumes, or excessivethickness. Parts often need to be scrapped; they cannot be repaired.

In vacuum bag processing, one side of the structure is tooled and theother is defined, at least in part, by the bagging materials used overthe layup. Bag side roughness and mark off is a common problemexperienced with prepreg processing and bag liquid molding processes.Cauls and intensifiers are often used on the bag side of the laminate toimprove surface finish. These surface enhancements, however, are notparticularly effective in the Palmer or Seemann process because of theflow medias used. The coarse knotty knit flow media and the bag offsetmaterials described in the Seemann process result in bag side mark offon the parts even in the presence of peel ply separator. Mark off occursbecause of localized high pressure at the knit knots or bag offsets withrelatively low pressure in surrounding areas. The uneven pressuredistribution produces a relatively lumpy bag side surface. Fiber volumeand fiber content varies.

Palmer uses glass bleeder cloths to form part of his flow media pack.Layers of dry glass cloth tend to bunch, buckle, and bridge under vacuumcausing severe mark off problems even on simple geometric partconfigurations, not to mention the complications that arise in morecomplex assemblies.

Used in the Seemann process to achieve rapid lateral flow, thick flowmedia and bag offsets create relatively large volumes that willultimately fill with waste resin. In Palmer's process, the flow media,the thick glass packs, and also the glass bleeder diapers waste resin.Palmer also loses resin when it flows beyond the end of the infusion andwets into the breather, as we discussed. We seek to minimize resinwaste.

In our preferred process, resin losses in the flow media are reducedbecause of its low profile and relatively small open volume. Our processalso allows for simple purged resin reclamation and recycling withoutthe risk of bag relaxation or the need for continuous resin purging withfresh resin to infuse difficult preforms. Our preferred processconserves resin and reduces cost measurably when working with expensiveresin systems, as is common for aerospace applications.

Neither Seemann nor Palmer describes how to produce complex assembliessuch as contoured skins with blade stiffeners, where the plumbingrequirements are complex. Each stiffener requires an active vacuum lineattached at the top of the stiffener to draw the resin up into thestiffener. When there are a large number of stiffeners, the plumbingquickly gets complicated. Each connection requires flawless seams withthe bag to preserve vacuum integrity. In our process, some stiffenerscan be effectively infused without using active vacuum lines. Inclinedinfusions where the resin is introduced at the lowest point and pulledup the preform to the highest point can effectively wet out stiffenersrunning in the flow direction and in some cases other directions asdemonstrated in our TYCORE™ sandwich panels.

Our process also can install passive vacuum chambers (PVC) inside theinner bag. Perforated tubes, spiral cut tubes, springs or other opencontainers are placed above stiffeners or other areas where flow isdesired (E, FIG. 8 or 9). The resin or liquid is pulled into thesechambers until they fill. The PVCs also provide some purging capabilityfor removing air from preforms.

By “wet out” we mean infusion of the desired amount of resin into thepreform to achieve the desired fiber volume in the composite.

The Seemann and Palmer processes can produce parts of almost unlimitedlength but are limited with respect to part width. Seemann's process cangenerally produce wide simple shells because Seemann uses flow mediahaving high permeability and bag offsets. Palmer's process is somewhatmore limited because it relies on an edge feed method and uses flowmedia of lower permeability. At some width, however, both the Seemannand Palmer processes require additional feed lines to reduce resin dragand pressure drop in the system, especially where flow on a skin isinterrupted with stiffeners. Stiffeners create choke points when theresin is flowing transverse or at an angle relative to the direction ofthe stiffener. Because of tooling constraints, dimensional controlrequirements, and shape discontinuities, care must be taken to placeflow media materials in stiffener locations properly.

A variety of dry preforms are available for constructing infusedcomponents. Both Seamann and Palmer use dry preforms. The optionsinclude standard weaves, warp knit materials, 3D braids, 3D wovenmaterials, stitched preforms, Z-pinned preforms, continuous strand mats,and chopped fiber preforms. Many dry preform materials are fragile,easily distorted, damaged, or frayed from simple customary manufacturingoperations. Distinct ply dropoffs, part tailoring, and net shapes aredifficult to achieve in complex finished parts made from dry preforms.Dry preforms also tend to have excessive bulk for layup of complexshapes where bulk must be minimized to eliminate wrinkling and baggingissues. To compound the problem, layers of the dry materials cannot bedebulked and consolidated effectively because of their poor adhesion toother dry plies or to other materials. Offline detail preformfabrication is ineffective. These characteristics make dry preformsdifficult, if not impossible, to use in many complex applications.Therefore, tackifier or binder technologies for treating dry preformswith resin necessarily become key elements of almost any liquid moldingtechnology system. The binder must not restrict resin flow or preformconsolidation, must be compatible with the infusion resin, and must notproduce loss in strength. The process of applying binder or tackifierproduces a preform similar to those used in conventional resin transfermolding.

In our preferred process, again, we have developed a unique sprayimpregnation process to apply the binder or tackifier to the dry fiberpreform to produce high tack with low binder content. Desired bindercontent ranges from about 1 to 10 wt % (i.e., by weight) but typicallyare from about 3-7 wt %. The desired weight percent depends on theweight and thickness of the preform and the natural or inherent degreeof tack in the binder.

Adding solvent to the semi-solid viscous liquid molding resins useful asa binder produces binder solutions suitable for spraying. The resinsshould have room temperature tack and be compatible with the infusionresin selected. For cyanate ester infusion resins, we typically useCIBA's M-20 semi-solid cyanate ester resin that is extremely tacky atroom temperature. Some semi-solid resins with no room temperature tackcan be used if they develop tack when heated, for example, 5250-4-RTMbismaleimide resin. The solutions sometimes require catalysts for resinactivation. For more latent spray formulations, the catalysts can beeliminated or reduced from the mix to allow higher temperature vacuumdebulk operations without adversely advancing the degree of cure of thebinder. Binder contents can be increased at ply edges to provide greaterdimensional integrity and less edge fraying. The binder might alsoincorporate thermoplastic or rubber toughening agents for improveddamage tolerance and ballistic survivability.

The preferred binder formulations typically have high or very high resinsolid contents of 80% by weight or more. The solvent or carrier can beMEK, MiBK, other organic solvent capable of dissolving the semi-solidresin, or, possibly, water. Solvent volatility can be altered and usedto control or to adjust tackiness and to change drying time and thecuring temperature or curing cycle profile. High solids content, highspray viscosities, and dry film spray parameters are used in conjunctionto form uniformly distributed small resin spots that rest on the exposedsurface of the preform. The preferred spray parameters minimize solventemissions, increase transfer efficiencies, allow automation, andmaintain maximum preform tack with the least amount of deposited resinand loss in preform permeability.

With these preforms coated with binder (i.e., tackified), we havedemonstrated the ability to produce complex structures such asintersecting blade stiffeners, Pi (π) joint stiffeners, and complexcontoured skins with curved blade stiffeners. The binder technologymakes it possible to net mold certain features such as blade stiffeners.Vacuum bag, room temperature debulking can produce soft, pliable,tackified preforms. Heated vacuum debulking can produce semi-rigidpreforms suitable for precision trimming to close tolerance.

With the Seemann and Palmer processes, the resin must be gelledimmediately after part infusion. If you leave the vacuum active on thepart, resin from the source is pulled through the preform during thegelling. The resin supply must remain connected to prevent the part frombeing depleted in resin, For most resins, gelation is initiatedthermally. Heating the part to gel the resin in the preform also heatsthe bulk resin which can lead to a hazardous exothermic condition,including evolution of toxic smoke.

If you close the vacuum and feed lines for the bulk resin prior toheating the preform, leaks might cause air to bleed into the breather.This bleed often produces defective parts that have high void content.The part may swell to create low fiber volume components or, moretypically, ones having voids or porosity. The Palmer process requiresalmost instantaneous gelation, but excessively rapid gelation oftenproduces brittle resin matrices. Many common resins, such as lowtemperature cure epoxies for high temperature applications, cannot begelled rapidly.

SUMMARY OF THE INVENTION

The present invention is a liquid molding process and system forproducing quality composite structures at low cost. It falls within thecategories of resin transfer molding (RTM), particularly vacuum-assistedRTM (VaRTM). The simple tooling, minimal capital requirements, batchprocessing capability, high yield, and capability to mold complex shapesmake the process attractive. For making aerospace structure, it promisesto be an economical process, especially suited for large structures,including wing boxes and the like. The present invention dovetailsnicely with other enabling technologies such as stitching, Z-directionreinforcement (Z-pinning), electron beam curing, 3-D weaving, and lowtemperature curing. It does not require an autoclave, matched tooling,or large presses.

High vacuum integrity with a double bag system of our design helps toyield high quality composites consistently with low void content,minimal surface porosity, excellent thickness control, and high fibervolume fractions. The double bag improves stiffness of the baggingmaterial to avoid relaxation behind the wavefront, thereby permittingthe infusion of void-free composites having the high fiber volumesdesired for aerospace applications. Controlling relaxation effectivelymeans that we can use a higher differential pressure (DP) as the drivingforce for resin transfer. We can infuse faster or can use more viscousresins because of the larger driving force. We desire fiber volumefractions in excess of 50%.

Resin wave front control produces clean infusions without surfaceporosity, voids, dry spots, or resin rich zones. Seemann uses thickcoarse flow media to direct resin to the underlying preform and bagoffsets. The media and offsets create a highly permeable space for rapidresin migration laterally in the bag. The speed of infusion, however,can lead to trapped air or surface porosity defects or voids as theresin percolates down through the thickness of the preform. Lateral flowcan exceed downward wetting of the preform, trapping air in pockets. Airtrapped behind the wave front becomes difficult to remove from theinfused part. Bubbling as air escapes can make it difficult to establishan end point for the infusion.

The key to successful infusions is not the speed with which the preformis infused, but rather the quality of the infusion. Maintaining acontrolled wave front with lower permeability flow media over thepreform gives cleaner infusions. The flow media we prefer to use shouldallow the resin to flow laterally slowly enough that the resin canuniformly drop down through the preform to wet out and completely fillthe preform with a wedge shaped flow profile. In a controlled flowfront, the resin front on the bag side of the preform is only 2 or 3inches ahead of the resin front on the tool side of the preform assumingflow media is placed only on the bag side of the preform and infusionincludes lateral flow through the media followed by downward flow tofill the preform. We prefer to control the relative permeability of theflow media to that of the preform to achieve this orderly, albeitrelatively slow, infusion.

Our unique, open weave, TEFLON impregnated glass flow media (Taconic7195) controls the flow front because it is thin, has modestpermeability, and its fill fibers form flow weirs. Besides controllingresin flow, the media works to solve a number of other issues, Thismedia can withstand exposure to temperatures up to about 600° F. and ischemically inert. It is free of contamination and has excellent releaseproperties. It is readily available, has relatively low foreign objectdefect potential, and minimizes bag bulk because it is low profile. Itreduces or eliminates mark off on the bag side of the laminate with itsstiff but pliable nature.

One option to achieve improved flow control uses bagging materials withhigh elongation (over 500%) and relatively low modulus, such asSTRETCHLON 700 polyester and STRETCHLON 800 nylon bagging films. Highelongation bagging materials make it easier to bag simple and complexpreforms with relatively few bag wrinkles. Preform areas under bagwrinkles tend to have relatively high permeability and can result inundesirable resin channeling along those bag wrinkles. Therefore,minimizing bag wrinkles with high elongation bagging materials improvesflow front control.

Another option uses gum rubber seals around the periphery of the part.With no edge seal or solid edge seals, channeling often occurs at theedges of the preform because of the high permeability that exists in thegaps typically found between the preform, bag, and solid seal. Usinghigh elongation bags and gum rubbers seals together with/or withoutthermal vacuum cycles, a tight seal between the edge of the preform,bag, and seals can be achieved. The gum rubber moves viscoelastically tofill in all the gaps that otherwise exist at the irregular edge of a dryor binderized preform (i.e., a preform having fibers coated with orcontaining binder or tackifier). Gum rubbers seals have been found to beparticularly useful when dealing with thick preforms where one has largebag discontinuities at the edge of the part. Bag bridging at theselocations allows excessive channeling. Gum rubber seals, however, workto seal the edge effectively, ease the bag transition, and reduce theeffects of edge tapering on the preform from bag stresses.

A double ended vacuum pull off technique successfully defeats channelingthat may occur for any number of reasons. If resin channels along oneedge of the part, the potentially “fatal” problem can be simplycorrected by clamping the vacuum tube on the channeled side andcontinuing the vacuum infusion with the opposite vacuum line in anactive mode.

Another method proven to defeat channeling is to infuse preforms in aninclined orientation with the resin being fed at the lowest elevationand moving upwardly through the preform with vacuum through portslocated at the highest elevation. With this method, gravity helps tomaintain a constant fluid level in the preform and resists resin flow,at least partially. Some preforms, such as multi-axial warp knitpreforms with bundled unidirectional fibers can have naturally occurringpermeability variations that can cause poorer flow control than inpreforms using more consistent materials such as 5 and 8 harness satincloth.

A thermal vacuum cycle used prior to infusion also minimizes channeling.Here, the preform gets debulked (i.e., compressed while having airremoved from between plies) to a thickness within about 10% greater thanits final thickness. Likewise, the bag's modulus drops at elevatedtemperature where it more easily elongates. As the bag elongates, itfits better and better to the underlying preform material, eliminatingall but the most severe of bag bridges. In cases of severe bag bridging,as for example at discontinuities around tooling elements for bag sidestiffeners, we use gum rubber seals either between the inner and outerbags or directly inside the inner bag at the discontinuity to helpbridge the gap. Eliminating bag bridging avoids channeling and resinrich areas that would develop at the bridged sites.

We can reduce or essentially eliminate bag mark off with the use ofsemi-boardy, closely woven, TEFLON impregnated fiberglass materials suchas Taconic 7195 or ChemFab CHEMGLAS 1589 as a separate ply and flowmedia. The low profile minimizes bulk and allows better contouringrelative to several layers of glass cloth materials. The uniform, closeweave construction of our flow media results in more uniform pressureapplication across the preform relative to knotty knit materials or bagoffset materials. The low profile and weave uniformity of our flow mediaalso makes it possible to use cauls or intensifiers effectively abovethe flow media to enhance part surface smoothness. The semi-boardynature of the flow media works to buffer bag and breather wrinkles fromtransferring to the infused part even in the absence of pressureintensifiers or caul plates. When our flow media is used in conjunctionwith vacuum thermal cycles, high elongation bags, and gum rubber sealsaround major discontinuities, mark off is substantially eliminated evenon complex parts. Mark off can cause a local weakening of the compositecaused by stress concentration.

In our process, an Airweave N-10 breather, between our inner and outerbag has a tendency to bridge over part discontinuities and to fold inareas of excess bulk. To achieve optimum fit between the part and thepreform, the breather and the outer bag are placed over the inner bagwith vacuum to seat the breather temporarily. The outer bag and thebreather are removed. The breather usually is then, cut and darted toallow a perfect fit. The breather, elastomeric materials that form theouter bag, and the breather network can usually be reused.

Therefore, preferred embodiments of our process produce composites withlow void contents, minimal surface porosity, excellent thicknesscontrol, and high fiber volume. The preferred process provides highvacuum integrity, eliminates resin channeling and poor wave frontcontrol. It greatly reduces bag side mark off. It reduces the plumbingcomplexity and improves manufacture of wide composites. Finally, ourpreferred process reduces resin waste.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the preferred features of our double bag vacuum infusionsystem, with the bags partially cut away.

FIG. 2 is typically cross-sectional view of our double bag vacuuminfusion system, taken generally along line A-A of FIG. 1.

FIG. 3 is an isometric of a preferred conformal tube fairing.

FIG. 4 illustrates a system for converting a vacuum line into a feedline without introducing air into the infusion.

FIG. 5 is a robotic spray system for applying a binder or tackifier to apreform.

FIG. 6 is an isometric of a preferred clamp for restraining the vacuumtubes to assure vacuum integrity.

FIG. 7 shows the typical plumbing used for infusing an I-beam stiffenedpanel.

FIG. 8 is a bagging schematic cross-section of the infusion of an I-beamstiffened panel shown in FIG. 7.

FIG. 9 illustrates typical plumbing for infusing an intersecting bladepreform.

FIG. 10 is a bagging schematic cross-section, similar to FIG. 8, of theintersecting blade preform infusion.

FIG. 11 is a bagging schematic cross-section for a truss stiffenedsandwich panel.

DETAILED DESCRIPTION

Our liquid molding process and system produces excellent qualitycomposite structures at low cost. Simple tooling, minimal capitalrequirements, batch processing capability, high yields, complex moldingcapability, and other processing features make it an extremelyeconomical method for composite fabrication. In addition to theprocess's affordability, it dovetails beautifully with other advancedcomposite technologies, such as stitching, Z-pinning, electron beamcuring, 3-D weaves, and low temperature curing. The preferred process isespecially suited for making large structures. Such structures have150-200 square foot area or more, such as a wing box, a bus body, or aboat hull. The process is also particularly suited for making largestructures having very complex stiffened assemblies (common to aerospaceto maximize the strength-to-weight ratio), and other unique productsthat are difficult, if not impossible, to make using known compositeprocessing technologies.

The preferred process of the present invention strives to:

-   -   1. Significantly reduce composite fabrication expense by using        lower cost raw materials, allowing part integration, reducing        capital requirements, lowering tooling cost, and quickening        cycle time;    -   2. Reduce worker exposure to hazardous materials;    -   3. Maintain high fiber volumes necessary for aerospace        composites and achieve the quality associated with current        prepreg processing techniques;    -   4. Enable the development of unique, previously unachievable,        advanced composite structures; and    -   5. Be easily deployed at practically any desired location with        minimal investment.

One embodiment of our double bag vacuum infusion process is illustratedin FIGS. 1 and 2. Advanced processing concepts are shown in FIGS. 3through 6. A double bag process improves vacuum integrity (desirable forlarge infusions) and reduces movement of the bagging material away fromthe preform behind the wavefront (i.e., “relaxation”), as sometimesoccurs with a single bag. The basic processing steps are:

-   -   1. Tool Selection and Preparation    -   2. Preform fabrication    -   3. Bagging and Plumbing    -   4. Vacuum Dry Out (Optional)    -   5. Infusion of the Preform    -   6. Resin Cure    -   7. Demolding    -   8. Postcure (Optional)    -   9. Trim and Inspection        We will discuss each of these steps separately in some depth.

Tool Selection and Preparation

Metal, composite, monolithic graphite, plastic-faced plaster, wood,foam, elastomers, modeling board, glass, or other materials for thetooling can provide the necessary vacuum integrity. Leaky tools areunacceptable because air will enter the preform and resin during theinfusion step. The resulting parts will have porosity or voids.Materials that are susceptible to leakage must be adequately sealedprior to use. The tools are typically single sided, but matched toolscan also be used. Parts can be tooled to the inner moldline or the outermoldline in male or female tools. We prefer female molds tooled to theouter mold line to provide a better surface finish. Molds of this typeallow cocuring processes for including internal stiffening elements,such as longerons, frames, spars and other features, into the structurewith minimal tooling requirements. Outer mold line tooling of this typealso allows molding of sills for access doors into the skins at thedesired locations.

Tooling details for incorporating internal features into the molded partare typically achieved using blocks of aluminum or other suitablematerials that sandwich stiffening elements desired for the preform.These tool details even conveniently allow consolidating and nettrimming of some of these preform features before installing thefeatures into the mold. Being able to consolidate these stiffeningelements ahead of time eases the preform layup, reduces overall cycletime, and permits precision features to be made without risking damageto the underlying skin.

Split elastomeric hat mandrels, used to produce contoured blades, can bemolded in our process. The flat hat mandrels are cast or water jet cutfrom a slab of rubber. Because the mandrel halves are made fromelastomeric material, the flat sections can be forced to twistingcontours. With the hat mandrel sections sandwiching the preformmaterial, the assembly can be forced to the skin contour in a way thateliminates gaping. This technique effectively resolves some assembly orjoining problems that can be experienced with more rigid, precisionmachined, metal tooling.

Although at least four times more expensive than similar aluminumtooling, PYREX glass project plates and tooling bars allow direct visualobservation of the resin flow front as the preform is being infused. Theleading edge of a resin wave front has a low angle tapered cross-sectionthrough the preform thickness. The infusion process goes through acyclic fill and drain process if the flow rate in the vacuum tubes isnot regulated to a low rate prior to final tube closure. Glass toolingis valuable for studying the infusion process and learning to controlit, because the tooling allows visual inspection throughout the process.Glass tooling, however, is likely impractical for many productionprocesses.

When producing complex co-cures, the internal tooling for the stiffeningelements must be precisely located in the mold because of typicalinterface requirements with joining hardware. Several techniques can beused for precisely locating the tooling, including removable tool stops,pins in offal regions or bolt locations, alignment guides, or high powerrare earth magnets for holding the tooling in place. Complex co-curescan be produced with excellent dimensional control of the various partfeatures.

When preparing a tool for part layup and bagging, we apply FREKOTE 700NC or another suitable release agent to ensure that the molded part willnot stick to the mold. We locate the inner and outer bag seals, whichmust be clean, free from residue buildup from previous runs, and mostimportant, protected from release agent contamination. To protect theinner and outer bag seal locations on the tool from release agentcontamination, we apply one-inch-wide, pressure-sensitive tape onto thetool in the seal locations as a mask prior to applying the release coat.Once the base tool is released, the pressure sensitive tape is removedto expose contaminant free seal locations. Solvent cleaning and/or lightabrasive honing might be needed for the tool surfaces after severaluses.

On high gloss composite tools, we prefer to abrade the tool in the seallocations lightly to achieve better sealant adhesion. We can also usereverse masking techniques, if desired, to abrade the tool only in thebag seal locations.

To minimize swirling patterns in the release coat and possible releaseagent transfer from the tool to the molded part, the release agentshould be hand wiped until all solvent has flashed. Wiping avoids the“coffee spill” effect where release particles aggregate at the edges ofthe streaks as the solvent flashes off.

Preform Fabrication

Preform Material

Our process can be used with essentially all preform materials,including quartz, PAN-based carbon, pitch-based carbon, glass, siliconcarbide, boron, organic, metallic, ceramic, and other fibers. Swirlmats, oriented chopped fibers, chopped fabrics, unidirectional warpknits, traditional bidirectional weaves, tri-axial fabrics, multi-axialwarp knits, 2D and 3D braids, 3D woven material, dry or binderized(i.e., tackified) filament windings, tow placed isogrids, hybrids,stitched reinforcements, and Z-pinned reinforcements are just some ofthe preform possibilities. Sandwich structures with foam, stitched foam,potted honeycomb, film adhesive sealed honeycomb, and syntactic foamcores can also be produced. It may also be possible to infuse structureswith properly treated metallic inserts to produce hybrid laminates, suchas Boeing's Ti/Gr materials. Perforated or non-perforated etchedtitanium foils can be interleaved with composite preforms to make highload bearing locations where composites might otherwise fail. Highstress composites, such as helicopter bulkheads or pyrotechnicallydeployed missile wings, are examples where local or isolated titaniuminterleaving might be used to advantage.

We can also integrate specialized materials into the structure, such asarc sprayed coatings, rain erosion materials, conductive ground planes,conductive knits, resistive cards, appliqués, microchips, MEMS,ceramics, antennas, sensors, or films.

The Need for Preform Binders or Tackifiers

Handling, cutting, forming, and consolidating dry fiber preforms intoprecision structures can be difficult. The degree of difficultyencountered depends on the starting preform material, the layup process,and the shapes being produced. Some stable preform materials (such as 3Dweaves, stitched preforms, multi-axial warp knits, heavily sizedfabrics, and tight thin weaves) can be used in simple structures withoutbinders or tackifiers. Other preforms (such as 5HS weaves, 8HS weaves,open plain weaves, unidirectional knits, and chopped oriented mats)usually are stabilized with binders and tackifiers to eliminate fiberloss, to allow automated NC cutting, to create crisp trim lines intailored layups, and to prevent excessive material distortion fromnormal handling.

The layup technique also plays a role in determining the need fortackifiers. Filament winding cylinders and/or pressure vessels with dryfibers would result in stable preforms that could be successfullyinfused. Braided tubes and other shapes would have stable preforms inmany cases without binders. Wound or fiber placed isogrids would alsolikely be stable dry preforms. Hand layup of preforms generally requiresbinderized material as does complex winding or braiding of noncircularclosed bodies.

Almost all preform materials can be used without binders to producesimple flat panels or slightly contoured shells without ply tailoring.Complex shapes such as radomes, tailcones, integrally stiffened shells,deep structures, multi-directionally stiffened components or highlytailored structures that are built, require tackifiers for materialadhesion to the tooling, enhanced consolidation or debulking, improvedtrimming, and better dimensional control.

Novel Spray Binderization Materials and Processes

Our preferred binder materials and processes evolved from previous workto produce specialty prepregs using a “Spray Impregnation Process.” Toadd binder to the cloth, the fibrous material is rolled out on tableswith an embossed polyethylene liner underneath to prevent contamination.The cloth is generally aligned so that the warp and fill fibers arestraight and orthogonal to one another. Binder solution is then sprayedon the exposed side of the cloth, although both sides can be sprayed ifdesired. The spray can be applied from a hand held gun or from arobotically mounted gun for more precise deposition. The sprayparameters are tuned so that fine, uniformly dispersed beads of resinare deposited on the surface of the preform, while attempting tominimize resin wicking into the preform. Having resin beads on thesurface of the cloth rather than a uniform distribution of resin in thepreform maximizes interply and tool adhesion while minimizing losses inpreform permeability, deposition time, and binder content.

Our preferred binder solution is typically a solvated, semi-solidpolymer that is compatible with the resin to be subsequently infusedinto the preform. These semi-solids typically have ambient viscositiesbetween 250,000 and 1,000,000 centipoise. The semi-solid material shouldhave high tack at room temperature or should be able to develop tackwhen gently heated. Furthermore, the semi-solid should liquify withoutsignificant additional cure when heated during a vacuum, dry outprocedure. Liquification of the applied binder with heat allows thebinder to wick into the preform when tack is no longer important toachieve appropriate positioning. The wicking action of the binder intothe preform while under bag pressure allows further consolidation of thelayup. If the resin does not appreciably advance its degree of cureduring the heated vacuum dry out procedure or during possible elevatedtemperature infusion operations, it may be able to bond chemically withor be dissolved into the infusion resin. Binder systems whichsignificantly advance (i.e., either partially cure or begin tocrosslink) prior to infusion can only form relatively weak mechanicalbonds with the infusion resin. Binders with high degrees of cure havebeen shown to reduce some composite properties by as much as 10%.

We prefer Cytec Fiberite's 5250-4-RTM BMI (bismaleimide) resin as abinder for 5250-4-RTM BMI infusions. Such infusions, however, requirehigh temperatures which increases their difficulty. We prefer CIBA'sM-20 cyanate ester (CE) semi-solid resin as a binder for infusions withcyanate ester materials such as Bryte Technologies EX-1545, EX-1545-1,and EX-1510. For infusions with low viscosity epoxy based resins, weprefer epoxy based semi-solid binders such as PR 500, 3501-6, 3502,977-3 and FM 300.

A typical catalyzed formulation of the M-20 cyanate ester bindersolution includes:

M-20 semi-solid 78.597% by weight Cobalt Acetyl Acetonate (CoAcAc) 0.086% by weight Dinonyl Phenol  1.572% by weight MEK 19.745% by weight

To make the M-20 binder solution, the CoAcAC, dinonyl phenol, and asmall amount of MEK is mixed for several hours with a magnetic stirrerin a sealed container. The extended mix time is required to dissolve theCoAcAc into solution. This organo metallic salt has relatively lowsolubility rates and requires significant time to dissolve. Thesemi-solid M-20 resin is then heated in a convection oven to between 120and 150° F. At these temperatures in the uncatalyzed condition, the M-20becomes fluid enough to dispense easily from its container withessentially no resin advancement in the degree of cure. Once the resinis dispensed, the bulk of the MEK solvent is then added to the M-20resin. The resin is dissolved into the MEK in a sealed pneumatic mix potthat prevents solvent flashing. Once the M-20 is uniformly dissolved,the catalytic solution of CoAcAc, dinonyl phenol, and MEK is added tothe base mix. The solution is then mixed in the same sealed containerfor several hours to blend and further dissolve catalyst whilepreventing solvent flash. The resulting solution is dark green and has ashelf life of at least one month, if it is stored sealed at roomtemperature without exposure to moisture. Failure to heat dispense theM-20 from its container can result in crystallization of the bindersolution. Crystallization is easily detected by observing that thesolution is a light pea green color.

A simplified and possibly improved M-20 binder solution appearstechnically feasible. Uncatalyzed M-20 cyanate ester can be thermallycured at temperatures above 250° F. Since the cyanate ester infusionresins are typically cured at 350° F. and higher temperature postcuresare often required, M-20 cure could be expected to cure in the absenceof any catalyst. The catalyst used for the cyanate ester infusion resinscan also probably catalyze the M-20 binder resin since similarchemistries are involved and the ratio of infusion resin to binder resinis relatively high. Infusion resin catalysis of the binder resin appearslikely since the infusion resins can dissolve the small M-20 resinislands and significant static mixing occurs as the infusion resinpercolates through the preform. The combination of thermal cure of M-20and infusion resin catalysis of the binder suggests that the bindersolutions can be formulated without the addition of the dinonyl phenoland the CoAcAc catalysts.

Eliminating these components from the binder solution has severalpotential benefits. The binder solution can be made significantly faster(in 1/10^(th) the time, perhaps) than it currently takes to produce thecatalyzed version. The resulting solution is likely to have a muchlonger pot life, perhaps as much as 6 months. The binderized preformsshould have extended working times at room temperature relative to thecatalyzed version and longer shelf lives in storage. The more latentmaterial would also tend to advance less than the catalyzed version whensubjected to thermal debulk cycles or thermal, vacuum dry out processes.The reduced degree of cure means the binder can more easily mix with theinfusion resin. Low degree of cure binders should never lock the preformin a poorly consolidated condition that would prevent high fiber volumesfrom being achieved during the DBVI process.

A typical formulation for the 5250-4-RTM BMI binder solution is:

5250-4-RTM BMI Resin 80% by weight MEK 20% by weightTo formulate this binder solution, the 5250-4-RTM BMI resin is heatedand dispensed typically from a 5 gallon can using a Graco hot meltdispenser. The resin is heated to between about 230° F. and 270° F.prior to dispensing. At these temperatures, the BMI will slowly advanceover a period of approximately 3 hours. Since the dispensing operationcan be completed in less than 15 minutes, insignificant resinadvancement occurs. Insignificant advancement also occurs during thethermal vacuum dry out cycle. Once the resin is dispensed into a vacuummix pot and has cooled slightly, MEK solvent is added to the resin. Theresin is dissolved into the solvent carrier using a pneumatically drivenstirrer in a sealed container. Pneumatic stirring is used to avoidpotentially explosive conditions that could result with electric mixers.Once the resin is dissolved, the BMI binder solution is ready for use.

Another approach for making 5250-4-RTM BMI binder solutions is todispense resin in a 1 gallon can or equivalent using the hot meltdispense technique. The resin is then cooled to room temperature forresolidification. Ceramic or stainless steel balls are then added to thecan. The can is sealed and placed on a rolling balling mill or in apaint shaker for a short period of time. The impact of the balls causesthe brittle semi-solid resin to pulverize into a fine powder with a muchhigher bulk factor. Milled resin can be kept sealed and frozen in thecan until needed for making binder solution or can be used immediately.Increasing the surface area of the resin allows the MEK solvent tosolvate the binder resin more quickly than is possible with simplepneumatic mixing of a large sections of bulk solidified resin withsolvent. Once the solvent is added, little additional mixing isrequired. The solution can be filtered through disposable filters toremove the milling balls and any other possible foreign objects.

Although the formulations given specify the use of MEK for dissolvingthe resin, other solvents or diluents can be used such as acetone,N-methyl-pyrrolidinone (No), methyl isobutyl ketone (MiBK), water,reactive diluents and others. Diluent changes can be used to modifyspray characteristics and increase room temperature tack of sprayed, lowtack, semi-solid binder resins. Solvent retention in the sprayed bindercan be achieved through the use of lower volatility solvents (i.e.,solvents with lower room temperature vapor pressure) or solvents whichhave higher affinity for the given resin material. The solvent retainedin the applied binder plasticizes the resin and typically increases roomtemperature tack characteristics of low tack resins. The solvent in thebinderized preform materials can be subsequently removed prior to resininfusion during the heated, vacuum dry out procedures.

The semi-solid polymer contents in the binder solutions will betypically in the vicinity of 70-90 wt %, and, generally, 80 wt %. Sprayviscosities range between 100 and 500 centipoise. Binder solutions areconsiderably thicker than many spray materials. The higher viscositiesand the high solid contents work to maximize transfer efficiencies,achieve droplet dispersion rather than fine misting, minimize solventemissions, and prevent resin migration and wet out of the cloth. As thesolutions travel from the gun to the surface of the preform, solvent(plasticizer) flashes and the remaining resin becomes more viscous sinceit has less plasticizer. The thick droplets cannot wick appreciably intothe cloth when they make contact. Transfer efficiencies are on the orderof 50-60% with typical air assisted airless spray processes. Theseefficiencies can potentially be increased over 90% with the use ofelectrostatic liquid spray technologies.

The binder contents on the cloth typically range between 1 and 10 wt %,although 3-7 wt % is more typical. Lighter weight cloths, such as 5HS,generally need higher binder content than thicker preforms, such asmulti-axial warp knits, because the tack is more related to surface areathan volume or weight of the preform. Preforms with a binder content inexcess of 15 wt % can experience wet out and create permeabilityproblems during resin infusion.

Preform materials are typically binderized in sheets. Once the majorityof the solvent has flashed after being applied, embossed polyethylenefilm is then placed over the binderized preform. The polyethylene sheetscan be cut manually with templates or with automated, numericallycontrolled knives on large vacuum beds to produce the required plies.The binderized material can be manually spooled on large diametercardboard tubes, sealed with Mil-B-131 bags, and placed in a freezermaintained at a temperature of about 0° F. Prior to removing binderizedmaterial from Mil-B-131 freezer bags for use, the material must bewarmed to ambient conditions to prevent water from condensing on thebinderized preform.

Individual plies of material rather than sheets of preform can bebinderized if desired to conserve binder. Precise fit for the pliessometimes can be better obtained in layup molds using dry cloth withoutbinder. Binder can be applied after molding to assist adhesion to theunderlying mold or plies. Using dry cloth is particularly helpful whereflat patterns have not been developed and trial and error assembly toclose tolerance is required.

The binder materials and processes were developed principally foroffline spray application on preform materials. However, the bindermaterials and spray application process can be used during preformlayup. If additional tack is needed in a given area, needed binder canbe applied locally. First ply preform layups on contoured tooling, plysplices with butt joints, embedded sensors, and holding stiffenerdetails in position are examples where on line, in mold application ofbinder is helpful. In dry wound or braided structures, the preform couldbe periodically tackified on the mandrels as the fibrous material isbeing built up. Doing so would eliminate the need to apply binder toindividual continuous tow material and would allow the tow material tobe used without freezer storage, shelf life limitations, or gumming ofdelivery equipment. Tow breaks on towpreg spools can be common withexcessive tack between the spool wraps. Some lesser degree of breakagemight also be expected with binderized tow on spools. So by applyingbinders to the tow material on the mandrel rather than to the towpackaged on the spool, fiber breakage can be avoided while stillproviding preform stability. Tow gaping voids experienced with towpregscan be eliminated when dry or binderized wound or braided preforms areinfused using the process of the present invention. Difficultiesassociated with wet winding and wet braiding are also avoided.

For robotic spraying of the binder onto the preform (FIG. 5), the bindersolution is charged to a pressure pot 501 with a disposable polyethyleneliner. The lid 502 is installed and clamped pressure tight. A fluiddelivery hose 503 is connected to the pickup tube 504 inside thepressure pot. Pressure regulated nitrogen or dry air is inject throughline 505 to pressurize the pot and force resin into the pickup tube andline. The pressure pot has pressure relief valves to prevent overpressurization and to bleed pressure from the pot for removing or addingresin. A regulator is installed near the gun 506 to control the fluidpressure being delivered. Controlling the fluid pressure at the guncontrols the volumetric flow rate through the gun's spray nozzle.Installing the regulator near the gun eliminates any pressure dropinfluence from hose length, hose diameter, or robot arm height. Nozzlecontrol is also needed to control flow rates. Slight manufacturingvariances in the nozzle orifice can result in different liquid flowrates. Nozzles of a given type are examined and screened for uniformityand are used exclusively for spraying the tackifier resin. Nozzlecontrol and fluid pressure regulation at the gun work in conjunction togive consistent and repeatable volumetric flow rates through the nozzle.The air assist atomization pressure through line 507 also is regulatedand controlled to give consistent spray dispersion from the nozzle.

The robot 508 carries the gun and is programmed to traverse across thepreform with a constant offset from the preform 509 and a controlledvelocity. The spray from the nozzle typically has a flat fan pattern.Most of the spray material is deposited at the center of the fan withtapering amounts delivered at the fan edges. To compensate for thisnonuniform distribution in the spray fan, the robot is programmed tooverlap adjacent passes to even out the distribution. Typical passindexing is ¼ fan width.

Coupling all the controls together results in consistent, uniform binderdeposition on the preform. Quality control tests are performed at thestartup of the process to insure the binder is delivered as desired.Noise variables can influence the binder contents realized. Tocompensate for noise, the robot speed can be adjusted. The robot canproduce sheets of binderized preforms up to 60 inches wide and 20 feetlong using an appropriate spray booth, robot arm reach, and robottransverse movement.

Large volumes of rolled or spooled broadgood preform materials could beeconomically binderized. A system with feed and take up rolls, a sprayzone, and a solvent flash zone can efficiently produce binderizedmaterials at low cost with poly or paper backed liners or possibly noliners.

Co-cured composite structures need to increase the pull off and shearoff strength values between the stiffening elements and the underlyingskin. With high strengths, “chicken fasteners” are usually not required.Eliminating fastener installations results in significant costreductions. High peel strength adhesives placed between the skin and thestiffening elements maximize skin-to-stiffener strength values. Tough,high peel strength binder resins likely can be developed for localizedapplication at these critical zones. The binders would emulate theeffect of the film adhesive but provide the necessary permeability forinfusion resin flow.

Most binder technologies currently use powdered resins that lack roomtemperature tack. Although tacky when heated, the pot lives of thesebinders are generally short at elevated infusion temperatures. Thebinders can advance so far in their cure that they cannot chemicallybond with or dissolve into the infusion resin, so the primary bondingmechanism tends to be mechanical rather than chemical. Powder binders donot lend themselves to uniform distribution on the preform. Powderadhesion to the preform can be relatively weak resulting in materialloss and foreign object powder debris in the layup area. Furthermore, itis difficult to control the actual quantity of powder deposited.Electrostatic powder deposition applies excess powder that must beshaken off the preform before use. Powder depositions trend toward anatural binder content level that is difficult to alter to more desiredlevels. In many cases, the low flow characteristics of these powderedsystems can impede the consolidation required to achieve “aerospacegrade” fiber contents. Low room temperature tack, low adhesion to thepreform, FOD potential, poor chemical bonding, inadequate binder contentcontrol, difficult automation, preform consolidation restrictions andother conditions make powdered binders much less desirable to use. Weprefer binder solutions and liquid binder spray techniques.

Preform Assembly and Build

In the hand layup process, a finished preform typically consists ofmultiple layers of fibrous materials cut, stacked in the desiredorientations, and debulked. Preform subassemblies can be manufacturedoffline for later installation in the final preform. The preforms can beassembled with dry fiber materials or binderized materials for improvedintegrity and consolidation. Finished preforms can be made from dryfiber materials, binderized fiber materials, or a combination of dry andbinderized material depending on the part requirements. The preforms canalso incorporate a wide variety of other materials including foams,honeycomb, prepregs, film adhesives, metals, ceramics, sensors, andother specialty materials.

Generally speaking, more care must be exercised when handling dry orbinderized preforms relative to traditional prepreg materials. On theother hand, dry or binderized preforms typically require fewer vacuumdebulking steps than prepregs. For simple geometries, the entire stackof collated preform material layers can be vacuum debulked once at roomtemperature. As the part complexity, contouring, features, and thicknesstailoring increase, additional vacuum debulking steps are requiredduring the preform collation.

Binderized materials are vacuum debulked at room temperature to produce“soft binder preforms.” Typically, soft binder preforms must remain onthe forming tool for shape retention, but sometimes can be precisiontrimmed for net molding operations. The consolidated “soft binderpreforms” tend to have some springback and tend to expand in thicknessslightly when removed from a vacuum bag. The amount of “springback”after consolidation is dependent on the binder tack, the binder content,and other factors.

Binderized materials can also be vacuum debulked at elevatedtemperatures to produce “semi-rigid preforms.” The stiffness generatedduring the heated debulk results from binder advancement in degree ofcure and/or solvent removal. Having additional stiff-ness and stability,these semi-rigid preforms allow simplified precision trimming. Detailscan be removed from the supporting tooling if handled carefully. Fewerdebulking tools are needed to mass produce dimensionally stablesemi-rigid preforms that can be stored temporarily. The semi-rigidpreforms remain permeable and can be effectively resin infused.

The final collated preform should, in most cases, be debulked prior tothe bagging and plumbing operations required for resin infusion. Thisfinal preform debulk allows the layers to flatten and grow in planewithout edge seal interference, and provides a better starting point forlayup of the inner bag components.

Bagging and Plumbing

Our preferred ‘double bag’ process results in superior vacuum integrity.The inner bag seal is covered by the outer bag. The inner bag can notpeel away from the inner bag seal as easily because it is locked intoposition by the outer bag. Furthermore, the inner bag is completelyisolated and enclosed to protect it against damage. If a minor leakdevelops in the inner bag, the system will continue to perform sincevacuum within the outer bag will prevent air from entering the innerbag.

To achieve even greater vacuum integrity, both inner and outer bag sealsgenerally are taped down with pressure sensitive adhesive tapes toprevent bag peel. Heated, vacuum dry out process is sometimes used tocure the gum rubber seals partially offline prior to the liquidinfusion. Precuring toughens the seal. If a leak is detected prior toinfusion, it can easily be repaired without affecting the part. A sealfailure may occur unexpectedly during the initial heating when the gumrubber softens from the heat and before it has had sufficient time tocure significantly. Using a heated, vacuum dry out process beforeinfusing the resin, the tacky gum rubber seals are tested and curebonded to the bags so that seal failure is unlikely to occur during theliquid infusion. The curing of the elastomeric seals also makes themless susceptible to resin attack and possible contamination of the resinand preform. Besides improving seal integrity, the heated vacuum dry outprocess serves to remove any volatiles in the preform, seats the bags,and improves consolidation prior to the infusion step.

Simplified plumbing to supply resin and vacuum reduces vacuum leaks. Apreferable approach for porting is to deliver resin and vacuum to thepart with tubes that pass through the gum rubber bag seals. Withthrough-the-seal tube delivery, special fittings are not required and notool perforations are required which could limit tool use for otherapplications. A variety of tubing can be used. The tubing, however, mustbe chemically inert, able to withstand the thermal processing, notcollapse under vacuum, and provide an effective seal with the gum rubbersealant it passes through. The tubing should also be pliable enough tobe externally clamped repeatedly without cracking. It should havesufficient, repeatable memory to recover to an open position afterexternal clamps have been removed so that in line valves and additionalfittings are not required. The tubing should be clear (opticallytransparent) or at least translucent to allow direct observation of theresin flow. Polyethylene, polypropylene, nylon, and TEFLON tubing meetmost of the requirements, but we have found the TEFLON tubing to beoptimal for handling higher temperature resins that cure at 350° F. orabove. TEFLON tubing includes ETCFE, PTFE, FEP, and PFA types offluoropolymers. In our improved process, an economical chemical etch ofthe TEFLON tubes maximizes adhesion of the gum rubber seals to thetubes. Also we elliptically flatten the tubes in the seal locationsusing a high temperature thermal process and a gauged clamping tool tominimize possible leaks resulting from tube line pressures cuttingthrough the soft gum rubber seals.

External stresses on the resin and vacuum porting devices can also causeleaks. These stresses can be introduced during handling, clamping, orvalving operations. Our preferred process uses strain relief devicesapplied to the tubing as they exit the outer bag seal to protect thecritical seal-to-tube interfaces from excessive stress. Not even abusivetube handling will degrade the seal integrity.

Some liquids (i.e., resins) infused into the preforms are sufficientlyreactive to attack the inner bag aggressively when in direct contactwith the bagging materials. To prevent such attack and subsequent lossof vacuum, we place an inert barrier film, such as FEP A4000 or WL5200(from Airtech Int'1.) between the flow media and the inner bag. Resincontainment in the preform by gum rubber seals also simplifies layup ofthe barrier film and prevents inner bag attack at the perimeter of theinner bag where it is unprotected with the barrier film.

The high vacuum integrity achievable with our process helps toconsistently produce composites with low void contents, minimal surfaceporosity, excellent thickness control, and high fiber volume.

To make a simple flat panel, the vacuum bag debulked preform isoptimally bagged and plumbed as shown in FIGS. 1 and 2. Bag sealant 2and 3 for the inner and outer bags is placed on the tool 1 in areasmasked from release agent to ideally separate the bags about 3 inches.Likewise, inner bag seal and edge dam or preform separations should alsobe about 3 inches. This separation protects inner bag 62 (FIG. 2) frompossible bag attack with chemically aggressive resin systems when usedin conjunction with an inert barrier film liner 61. Many gum rubber bagsealant options, such as Scheene Morehead 5127, are commerciallyavailable. The paper backing on the top of the sealant is left in placeas a protection from contamination until the bags or tubing isinstalled. Care is taken when installing the sealant to avoid trappingair between the sealant and the tool. The sealant is rolled with the topbacking paper in place to improve seal seating while avoiding sealcontamination. Sealants must be used within allocated shelf life andkept from moisture or solvent exposure prior to use. Old sealants mayhave insufficient tack and may foam during part cure because of absorbedmoisture or solvents. Both phenomena reduce the critical seal integrityrequired.

A finely woven, porous peel ply material 59 is laid on the debulkedpreform 51 (FIG. 2). The peel ply material can be polyester, nylon,glass coated with a suitable release agent such as FREKOTE, orTEFLON-impregnated fiberglass. Typically, TEFLON-impregnated fiberglass,such as CHR3, is used, because it has superior release characteristicsand a fine surface finish. Peel ply materials are generally less than0.005 inches thick and are more typically 0.002-0.003 inches thick. Thepeel ply is generally terminated approximately ¼ inch from the two sidesof the preform that run parallel to the flow direction, although otheroffsets are possible. The peel ply on the resin supply and vacuum pulloff ends of the preform can be flush with the ends of the preform or canextend slightly beyond the end, in which case they are tucked under orover the spiral wound springs 5 and 6 (FIG. 1).

Flow media 60 is then laid over the peel ply 59 (FIG. 2). The flow mediashould be a low profile material that has high, uniform permeabilityrelative to the preform, such as open weave fiberglass, screeningmaterial, woven metallic screens, or chopped glass mats. The mediashould drape for contouring, have no contamination potential to theinfusion resin, provide adequate stiffness to prevent bag mark offs onthe part, and survive the required cure cycle. TEFLON-impregnated, openweave fiberglass materials such as Taconics 7195 or ChemFab CHEMGLAS1589 perform particularly well as a flow media material.TEFLON-impregnated fiberglass materials are approximately 0.020 inchthick, have a uniform woven structure, are chemically inert, and areresistant to temperatures up to 600° F. Their somewhat boardy (stiff)nature allows contouring and bending, but also serves to prevent bagmark off. Their permeability helps to control the infusion resin wavefront and prevents trapped void formation during the infusion, but cancreate problems for infusions with resins having very high viscositiesor limited working times before thickening. To increase the permeabilityof the media while still retaining the uniform feed and reduced mark offcharacteristics, higher permeability materials can be placed over theTEFLON-impregnated fiberglass. One option is to use coarser Taconics8308 or simply another layer of Taconics 7195 over the Taconics 7195 tocreate a more permeable flow media combination which will dramaticallyspeed infusion rates and allow processing of more viscous resin systems.

The flow media is typically placed directly over the peel ply and istucked on the ends under the spiral wound springs 5 and 6. Tuckingallows good resin feed coupling from the spring into the flow media andalso allows the spring to be easily removed from the part after curewithout causing edge delaminations. Flow media can be terminated on thepreform prior to the vacuum pull off spring to straighten the wave frontand to correct minor resin channeling. The resin velocity of the wavefront is reduced when it encounters the flow media dropoff because ofthe higher drag. This velocity reduction in the channeled areas allowslagging portions of the wave front still in the flow media to catch upwith the channeled zones so that the wave front proceeds uniformlythrough the flow media and approaches the spiral wound spring at thesame time.

Spiral wound steel springs (901 & 902, FIG. 9), used for resin feed andpull off from the ends of the preform, are typically placed on the longedges of the preform to minimize resin flow length through the preform.The springs are usually placed immediately adjacent to the edge ofpreform, but can also be placed on top of the preform at the ends ifmore offal trim provisions are given for the final part. The springs aresubstantially the same length as the edge of the preform. Becauselubricants are sometimes used to assist in winding the springs and toprevent corrosion, they must be cleaned with solvent, like MEK, in arinse tank; vapor degreased; or alkaline cleaned to prevent possiblecontamination. The springs can be annealed or made from softer metalssuch as aluminum or copper.

Because resin pull off should not exceed the resin supply, larger, moreloosely wound springs are typically used on the feed end of the preformthan on the vacuum end. Some typical spring specifications for feed andvacuum ends of the preform are:

Feed 3/8 in. O.D., 0.032 in. Diameter Spring Steel Wire, 10 Wraps/in.,Heat Treated Vacuum ¼ in. O.D., 0.032 in. Diameter Spring Steel Wire, 15Wraps/in., Heat Treated

Steel springs have been found to withstand the cure temperaturerequirements and do not collapse under the vacuum bag pressure. Thespiral configuration of the resin delivery and pull off devices producesgentle contoured preform edge. Drilled tubing, spiral cut tubing, spiralwound metallic ribbons, or small chains can be used instead of springs.Because of possible runaway reactions with catalyzed thermoset resin inbulk, the allowable diameters of the springs may be constrained fromgrowing beyond approximately ½ in. OD.

After the feed and vacuum pull-off springs have been installed, thepreform 51 (FIG. 2) is preferably sealed along the edge with a gumrubber sealant 52. The gum rubber should be deformable under bagpressure but yet have relatively little flow to prevent appreciablemigration into the preform and springs. The sealant typically used isAirDam 1B from Airtech International. This sealant is ⅜ inch wide andapproximately 3/16 inch thick. Fine, milled fiberglass is incorporatedinto the sealant for flow reduction. In some cases, we use the higherflow, AirDam I sealant. The sealant tape is stacked in layers in longstrips to approximate the thickness of the preform. Excessive sealantthickness may cause the inner bag to bridge the preform around itsperiphery. Insufficient sealant thickness may in extreme cases causeedge tapering in the preform from localized bag stresses.

Once stacked in strips to the correct thickness or height, the two sidesand top of the sealant is wrapped in an inert, lightweight, stretchable,release film such as A4000 or Wrightlon 5200. The exposed bottom allowsthe sealant to seat and to seal against the tool 50 (FIG. 2). With onlythree sides on the sealant covered with release film, it is able toexpand outward to form a tight seal with the irregular sides of thepreform. The intimate contact of the release film covered seal with thesides of the preform prevents resin channeling during the infusion. Therelease film constrains excessive sealant flow, prevents contaminationof the infusion resin and preform with sealant, and protects thetool-to-edge sealant interface from possible resin attack.

A continuous strip of sealant covered with a release film is laidtightly around the periphery of the preform to contain the infusionfluid until it is solidified during cure. Resin containment is essentialto prevent resin bleed and loss of hydrostatic pressure on the part,especially between the time that the vacuum and supply tubing is clampedand cut and the resin ultimately gels or solidifies.

Sometimes the inner bag 62 or the barrier film liner 61 fits around thepreform assembly to prevent channeling and provides the necessary fluidcontainment without the edge seals. We prefer using gum rubber edgeseals, particularly for thick preforms having large bag discontinuitiesat the edges, when the viscosity of the infused resin dropssignificantly during cure, or for fluid containment in inclined orvertical infusions.

Our process uses constant cross section, elastic conformal tube fairings(CTFs) with wide or large stiffened preforms. The low profile of thetapered CTFs minimizes bagging problems and potential mark offespecially when placed over a semi-boardy flow media. The CTFs can bemolded, cast in slabs, and water jet cut to create the desired bevels orcan be extruded to shape in simple molds or dies. Since these CTFs aremade from elastic materials, flat general purpose CTFs can be made tofit most contoured skin surfaces. A typical, preferred CTF 301 is shownin FIG. 3.

To augment the CTFs in producing wide assemblies, as the infusionproceeds, we convert vacuum lines into feed lines without introducingair into the bag and without creating mark off. A vacuum tube mounted ona stiffener, for example, can be used to infuse that stiffener andthereafter be converted into a feed line for the next infusion bay orsection (FIG. 4). This conversion between feed and vacuum uses a T or Yfitting. One leg of the T connection is connected to a resin supply, butcan be clamped or closed with a valve while the other leg is left openand connected to the vacuum drop out tank. Once resin begins to fill thevacuum line and clears the T fitting with no bubbles, the tube exitingthe bag is closed and the leg to the resin supply is opened. The supplytube fills with resin and purges air from all tubing. Once the tubes arefilled, the vacuum line leading to the drop out tank is closed and thetube exiting the bag is opened allowing a new resin supply to feed thepreform.

Tubing 8 is cut to the required lengths to connect the feed and vacuumpull off springs to the resin container source 14 and the vacuum dropout tank 9 (FIG. 1), respectively. TEFLON tubing made from FEP, ETCFE,PTFE, or PFA can be used for resins that cure between 250° F. and 600°F. Lower cost tubing is available for lower temperature exposures.

One end of each tube 8 is dipped in an etchant such as (Tetraetch fromGore Industries) to strip flourine from the TEFLON to create achemically active surface that will adhere aggressively to the sealant.After the etchant dip, the tubes are rinsed with water and dried. Theetched tubes remain chemically active for extended periods of time andcan be stored indefinitely if kept dry in sealed bags away fromultraviolet light exposure. Tubing inner and outer diameters for boththe feed and vacuum lines are typically 0.25 inch and 0.375 inch,respectively, but other diameter combinations are possible. Smallerinside diameter feed tubes may restrict resin feed to the preform.Larger ID tubes are more expensive and may result in uncontrolledexothermic conditions due to excessive resin bulk. Wall thickness of thetubing can be reduced, but tube memory for external valving suffers.Thin walled tubes collapse at elevated temperatures.

Once the tubes have been etched, distances from the end of the springsto the bag sealant locations are determined. The tubing is ellipticallyflattened in 1 or 2 inch wide bands where it will interface withsealant. Flattening the tubes in these areas increases the sealantsupport area and results in less cutting action through the sealant whenthe preform is bagged and the sealant softens. Heating occurs throughoutthe process during vacuum dry out, infusions, and cure. The tubes areheated to about 600° F. or higher with a standard heat gun and thencompressed in a vise. The vise has a stop set at a desired thickness toprevent tube collapse and to flatten the tubes repeatably to the samethickness. The tubes can be quenched in water after flattening and driedor air cooled to room temperature.

The etched ends of the tubes are preferably attached to the springs 5 &6 with pressure sensitive adhesive tape wraps, such as FLASHBRBEAKER,KAPTON or other high temperature resistant tape. On the vacuum end, eachspring is inserted a short distance into the joining tubes given themating diameters. The cut ends of the feed spring are bent toward thecenter of the spring to prevent possible bag puncture. When the tubesand springs are assembled, the backing paper over the bag seals in thetubing locations is torn and peeled back to expose the sealant. Thetubes and springs are seated directly over the bag sealant.

Tube strain relief devices 25 are installed. These U-shaped channels 25(FIG. 6) have a channel depth less than the outside diameter of theassociated tube to create a slight interference fit. Tube friction andthe large cross-sections of the tubes outside the channels preventsmovement in the critical bag seal locations.

We usually position a chemically resistant, lightweight, stretchableouter film 64, such as Airtech International's A4000 or Wrightlon 5200FEP TEFLON film, over the assembly to provide additional protection forthe inner bag 62 from resin attack, bag punctures, or resin leakage pastthe edge seal for infusions that occur at temperatures below about 350°F. Such a film covers the entire preform layup and edge seal and runs upclose to the inner bag seal. For lower temperature cures, polyethyleneor polypropylene films can be used.

If the resin is very aggressive, it can move beyond the edge of theprotective film and attack the bag unless additional strips or frames offilm are installed to the inside edge of the inner bag seal 56. Thesefilm strips typically overlap the base inert barrier film 61 by 1-3inches to restrict and to narrow the flow path for the resin to reachthe inner bag 62.

The inner bag 62 generally is a disposable or consumable film orelastomer. For long production runs, however, especially of complexstructures, conformable, premolded, reusable elastomeric bags made fromsilicones, fluorosilicones, Fluorel, nitrile rubber or other elastomericmaterials may be preferred. The bag 62 should be flexible and have highelongation capability with relatively low modulus to simplify baggingcomplex parts so that it can be vacuum formed around the preform evenwhere bridged. Bag bridging can occur over the preform atdiscontinuities. A low modulus bag reduces localized bag stresses on thepreform which otherwise can cause tapering, distortion, or preformdamage. Shaping the bag to conform with the contour of the preformminimizes resin rich zones in the finished parts, resin channeling, andedge tapering from bag induced stresses. Although stiff at roomtemperature, the film may become sufficiently flexible to stretch whenheated for vacuum dry out, infusion, or curing. Standard nylon bags forprepreg material processing at 350° F. will work, but are not optimalbecause of their relatively low ultimate elongation (200-300%) and highstiffness. STRETCHLON 700 polyester and STRETCHLON 800 nylon baggingfilms from Airtech International are superior, because they can stretchover 500% and have lower modulus than the standard nylon films. A VACPACpolyurethane film from Richmond Products is effective for lowtemperature cures below 160° F., because it has an extremely low modulusat room temperature and an ultimate elongation approaching 1000%. Forcures up to 600° F., special FEP based bagging films, such as ChemFab'sVB3, can be used instead of polyimide based films such as KAPTON orTHIERMALIMIDE. This FEP bagging material, which is etched on one side toimprove seal adhesion, has elongation over 500%.

For bagging of complex shapes, we stretch disposable films on mastertools at elevated temperatures. These stretched bags are easy to use andare superior to reusable, molded rubber bags in producing complexhardware.

Reusable, molded rubber bags are generally produced from gum rubbersheets that are spliced, seamed together over a master tool, and heatedto cure. The master can have patterns embedded or embossed on it tocreate channels or flow features directly into the bag, in the samemanner as Seemann suggests. Typically these rubber bags are treated toallow resin release after cure, to reduce bag attack, and to improvesealant adhesion in selected areas, especially around the edge.Composite materials can be incorporated into the bags in an attempt tocontrol shrinkage after cure.

Rubber molded bags are significantly more expensive than disposablebagging films, and the bags tend to degrade faster than one wouldexpect. Release liners are often attached to the molded rubber, but theycan disbond and create spider wrinkles on the bag molding surface. Seamfailures or bag tears can also occur.

Making the master mold for a rubber bag can be difficult and expensive.It must be sized to accommodate the high bag shrinkage that will occurafter cure. Even then, the bags continue to shrink over repeated curecycles resulting in poorer and poorer fits with the preform. Theassembly problem is compounded because the rubber bags have a muchhigher stiffness and loading on the relatively unstable preform thandisposable film bags. Force fitting the bags can actually result inpreform movement or damage. Properly cared for, the bags can onlywithstand about 100 cure cycles at 350° F., but they often fail in lessthan 10 cycles.

The inner bag 62 is cut oversize relative to the area contained insidethe inner bag seal 56. The protective paper is removed from the top ofthe inner bag sealant. The inner bag is seated on the sealant. Excessmaterial outside the periphery of the seal is trimmed and then apressure sensitive tape 58 (FIG. 2), such as FLASHBREAKER 1 from AirtechInternational, is used to tape the bagging film to the tool 50 toincrease vacuum integrity and to minimize bag peel.

After the inner bag 62 is installed, the feed tube is externally clampedwith pliers. The end of the feed tube is also temporarily closed offwith gum rubber sealant such as that used for the bag seals 55 and 56.Caps and ferrules are slid on the vacuum tubes 8. The ends of the vacuumtubes are slid into the fittings 16 installed in the vacuum drop outtank 9. The ends of the tubes are positioned at sufficient depth thatthe resin will drop into the disposable steel can 10. With the vacuumtubes at the proper depth, the caps are threaded onto the fittings toswage the ferrules on the tubes creating a seal to the vacuum drop outtank 9. Gum rubber sealant is wrapped around the fittings and caps toprovide extra seal integrity at the tube to drop out can joints.

A vacuum source 11 is connected to the drop out tank 9 to pull vacuum onthe installed inner bag 62. The vacuum line typically has quick connectfittings on both ends allowing it to be easily attached to the drop outtank and vacuum source. Once the bag is pulled tight with vacuum, thevacuum level is checked with a precision, vacuum test gauge or vacuumtransducer 12. If the part has an obviously low vacuum level asindicated by the gauge or signals from the vacuum pump, the bag andconnections are checked with a leak detector until the leak is found andrepaired. With a high efficiency vacuum pump, the vacuum level shouldconsistently exceed 28 inches of Hg. A vacuum in excess of 29 inches ofHg is preferred because it provides additional preform compaction.

AIRWEAVE N-10 or SUPERWEAVE UHT 800 Breather 63 (FIG. 2) is placed overthe inner bag and extends close to but not in contact with the outer bagsealant 55. The breather can be fiberglass cloth, fiberglass mats, flowmedia, or steel wool (for infrared flow front detection).

Outer bag 64 is installed over the breather 63 in a similar manner asthe inner bag 62. The outer bag is sealed to the tool 50 with thesealant 55 and pressure sensitive tape 57. To apply vacuum to the cavitybetween the inner and outer bags, a through-the-bag fitting 20 istypically used, although through-the-tool or through-the-seal tubescould also be used. The fitting is connected to a vacuum hose 13 that isalso equipped with quick connect fittings on both ends. The vacuumintegrity of the outer bag is checked in the same manner as the innerbag.

The inner bag vacuum level should equal or exceed the vacuum levelbetween the inner and outer bags so that a pressure is exerted on theinner bag from the chamber defined by the inner and outer bags. Thissituation occurs naturally when both the inner and outer bags areconnected to the same vacuum source. If the outer bag vacuum levelexceeds the inner bag vacuum level, the inner bag can be slightlydisplaced with less effective compaction of the preform.

Vacuum Dry Out

Once the preform is bagged and plumbed, we generally prefer to heat thepreform under vacuum to dry it. We can complete the drying step in aconvection oven, on hot plates, or on heated vacuum debulk tables, suchas those produced by Brisk Heat. Debulking compacts the preform, drivesoff volatiles that may be trapped in the preform or bagging materials,and uniformly disperses meltable binders into the preform throughcapillary action. The inner and outer seals are advanced through theirsoftest, critical stage where they are most likely to develop leaks. Thesealant bond to the bags improves through the rubber curing processproviding increased vacuum integrity. If a leak should develop duringthe critical soft phase in the gum rubber sealant, it is of noconsequence since the infusion resin is not present. Leaks discovered atelevated temperature can be easily repaired and confidence in the bagintegrity is enhanced. Drying helps to seat the edge sealants and vacuumforms the bagging materials to the preform materials creating a superiorfit. This fit improvement helps to eliminate possible resin channelingand bridging which would allow the formation of resin rich areas in thecomposite.

The preferred temperature cycle for the vacuum drying depends on thepreform materials in the bagging system. In a typical infusion, thetool, preform, and bagging materials are heated rapidly to 250° F., heldat that temperature for 1 hour, and cooled to the infusion temperature.For stitched preforms and 3-D woven preforms, the cycle is normally 2hours at 350° F. because of the hydroscopic nature of the organicstitching fibers and the water lubricants used in 3-D weave process withcarbon fibers. In some cases, such as preforms containing syntactic foamcores, heating is not allowed because it will melt and destroy the foam.Binders also play a significant role in determining the appropriate dryout procedure. For “soft” cyanate ester binderized preforms where thebinder is more soluble in the infusion resin, the cycle may be 1 hr at160° F. or ½ hour at 200° F. This cycle has a high enough temperature tomelt the binder material, to drive off residual carrier solvents, and towick the binder into the preform. This temperature is low enough toprevent significant advancement in the degree of cure. For “semi-rigid”preforms binderized with catalyzed M-20 cyanate ester, the cycle may be1 hour at 250° F. The heated, vacuum dry out cycles will vary, but, ingeneral, they have very positive effects on the overall infusionprocess.

Infusion of the Preform

The resin selected will dictate a number of the processing parametersselected for the infusion process, including the mixing and dispensingtechniques, infusion temperature, flow lengths, working times, degree oftemperature control, flow lengths, and flow media selection.

Preferred resins from a processing perspective have some or all of thefollowing characteristics:

-   -   a. Long pot lives at the infusion temperature (several hours or        more) to allow complex infusions and to ease timing constraints;    -   b. 1 or 2 part resins for mixing simplicity;    -   c. Can be mixed and infused at room temperature for out of oven        operation, operator comfort, improved process control, more        rapid processing with simpler and less equipment, allow        simplified real-time mass balances, and are more readily adapted        for recirculation techniques;    -   d. Have a viscosity in the range of 100-350 centipoise to allow        rapid infusion without channeling and reduced plumbing        requirements for large part fabrications;    -   e. Can be mixed in large batches without potential for hazardous        exothermic conditions;    -   f Are non-toxic and non-carcinogenic;    -   g. Can be stored at room temperature in the unmixed state to        eliminate need for thawing and freezers;    -   h. Are quick curing for reduced cycle time and greater oven        throughput;    -   i. Have low surface tension for improved wet out;    -   j. Do not release volatiles or other gases under high vacuum;    -   k. Have low heat of reaction to allow fabrication of thick        parts;    -   l. Cure at low temperatures to allow lower cost master        production tools;    -   m. Are compatible with bagging materials to minimize risk of        vacuum failure arising from the resin attacking the bags;    -   n. Are recyclable to reduce waste;

o. Are low cost with high temperature performance for demandingapplications;

-   -   p. Allow infusion at minimum viscosity in an end game scenario        to increase fiber volumes and reduce likelihood of loss of        hydrostatic pressure during cure;    -   q. Are amorphous (do not crystallize) during storage;    -   r. Do not gel prior to being heated to a maximum cure        temperature to eliminate tool stress on freshly gelled, complex        components    -   s. Possess consistent and repeatable gel times batch to batch.        Preferred resins include Bryte Technologies EX-1510 and EX-1545,        cyanate esters, ATARD Laboratories SI-ZG 5A anhydride based        epoxy, and Cytec-Fiberite's 823 epoxy. Preferred resins include        Bryte Technologies EX-1510 and EX-1545, cyanate esters, TARD        Laboratories SI-ZG 5A anhydride based epoxy, and Cytec-Fibente's        823 epoxy. Preferred resins are low viscosity liquids at room        temperature and consequently do not require heating for        infusion. Some resins such as Ciba Geigy's 8611 are thick        viscous liquids at room temperature (between say 1,000-10,000        centipoise) and must be heated to relatively low temperatures        (between 100-160° F. typically) to reach an acceptable viscosity        (less than 400 centipoise). Other resins such as 3M's PR 500 and        Cytec Fiberite's 5250-4-RTM are semi-solids at room temperature        and must be melted at relatively high temperatures to infuse in        an acceptable viscosity range. We shy away from resins that have        short pot lives excessively long cure cycles. We also avoid        resins that suffer micorcracking when cured.

Batch resin preparation for the low viscosity liquid materials isrelatively easy and simple. Components are accurately dispensed togetheron a precision balance, mixed for several minutes with an open airpneumatic driven stirrer, and then de-aired in a vacuum bell jar orequivalent for 5 to 10 minutes. If a vacuum mixer is available, theresin can be mixed and de-aired simultaneously. Viscosity checks can bemade with a Brookefield type viscometer.

For viscous liquids, the mixing must be performed on a hot plate withsubsequent de-airing in a vacuum oven to prevent cooling. The mixing,heating, and de-airing operations could also be performed in a jacketedvacuum mixer equipped with heating capabilities. Careful temperaturemonitoring is usually necessary to establish uniform desired temperaturein the blend and avoid possible hazardous exothermic conditions.

Semi-solid materials are most efficiently dispensed and heated using aGraco hot melt dispenser. These resins are normally de-aired in heatedvacuum ovens to minimize cooling.

Once the resin is mixed, heated (if required), and de-aired, the mass ofresin to be charged to the system can be determined. The amount of resinrequired for an infusion is typically the sum of the resin required tofill the tubes, preform, flow media, plus an excess working quantity ofbetween 400 and 1000 grams. The amount of excess required is dependenton the part configuration, the number of supply containers, and whetherrecirculation techniques are employed.

Because the double bag vacuum infusion process has a closed loop systemfor the resin, mass balances can be performed to estimate the resincontent or fiber volume of a given part prior to cure.

The weight of the preform can be measured directly or be estimated fromknown preform ply areas and nominal areal weights. With the resin andpreform weights and densities (ie., specific gravity), resin content andfiber volume can be easily determined.

Using precision balances on the feed container 14 and the drop out can10, mass flow rates, fluid velocities, percent preform fill, and fibervolumes can be determined at every stage of the infusion. For the dropout scale to work properly and have sufficient sensitivity, it must beplaced inside the vacuum drop out tank with vacuum seals to route thepower supply and feed back to the data acquisition equipment.

When the resin must be infused at elevated temperature onto a hot tool,the operation must be performed in an oven or the tool and/or resin mustbe heated. For ambient processing, the infusion can be performed inpractically any convenient location or in the oven directly. If theinfusion is performed outside the oven, processing capacity ismaximized. When infusions are performed outside the oven, it isimportant that the vacuum level in the outer bag does not decreaseduring the transfer from the infusion site to the oven for cure.

To begin the infusion, the end of the feed line 8 is cut with a tubecutter to remove the portion of the tube with the sealant plug. Anexternal constricting device is installed on the feed tube to reduce theflow rate of the resin in the initial phase of the infusion. Withoutthis feed constraint, the resin tends to shoot into the part too rapidlyand can trap voids behind the wave front that are difficult to remove.The end of the feed tube is placed in the feed container 14 near thebase and is secured. The feed can may be tilted at an angle with thefeed tube positioned in the lowest location to minimize the amount ofresin required to prevent air from entering the tube and bagged preform.To initiate flow, the sheet metal or welding clamp is removed from thefeed tubing. After a few minutes of infusion, the constricting device isnormally removed from the feed tube to speed the infusion rate.

The feed should be positioned below the lowest part of the preform.Positive pressure feed to the preform causes the inner bag to bulge nearthe feed spring. The vacuum tubes, on the other hand, should rise abovethe preform to help maintain hydrostatic pressure on the fluid and tominimize resin drain from the preform into the drop out can. Althoughpreforms can be infused successfully in the horizontal orientation, itis often preferable to infuse in an inclined or vertical orientationwith the feed at the lowest end and the vacuum pulled at the highestend. Inclined or vertical orientations tend to reduce channeling effectsin low viscosity resin systems and in preforms with high variations inpermeability. These orientations can also be used to eliminate otherwisenecessary plumbing.

As the infusion progresses, the infusion rate gradually slows. Theinfusion rate drops because of the increasing drag and pressure drop asthe fluid wets the preform. With a single layer of flow media, a singlefeed line or spring efficiently infuses 3-4 linear feet of preform inapproximately 1 hour. Flow lengths can be as much as 5-6 feet before anadditional feed line is required.

When resin reaches the vacuum end of the preform, the resin will fillthe vacuum tubes, and, then, cascade into the drop out can 10. Becausethe vacuum tubing has very high permeability relative to the preform,the fully wetted preform can be drained locally on the vacuum endresulting in loss of hydrostatic pressure on the resin in the preform.As the preform drains, the resin flow into the vacuum tubing decreases.At some point the resin feed to the preform exceeds the draining and thepreform will begin to fill again. The process of filling and drainingthe preform locally at the vacuum end of the preform will cyclerepeatedly unless active measures are taken. Bubbling in the vacuum tubeis often associated with this phenomenon. The rate of the bubblingincreases as the preform drains and decreases as the preform fills.

If the feed and vacuum tube lines are clamped and cut when the preformhas low hydrostatic resin pressure or is partially filled, the resultingpart will have surface porosity and in more severe cases, internalporosity. These defects will typically be located on the vacuum end ofthe preform. Consequently, it is essential to cut, clamp, and seal thelines when the preform is full.

The vacuum lines should be throttled or choked to a near closed positionuntil the mass flow rate of resin through the preform equals the massflow rate in the vacuum tube. In the choked condition, the resin feed tothe preform and the tubes exceed the pull off capability downstream ofthe choke point. Consequently the preform will completely fill. As thepreform fills, the mass flow rates in the preform will eventuallydecrease to match the mass flow rate beyond the choke point. Once thisquasi-steady state is reached where the feed and pulloff rates are equaland the preform is full, the bubbling action associated with the filland drain phenomena ceases. The vacuum tube between the preform and thetube choke point eventually fills with bubble free resin. The systemnormally reaches a quasi-steady state rapproximately 15 minute of chokeflow processing.

External clamps are typically used to throttle the flow, but internalplugs, nozzles, sintered metals/ceramics, filters, two position ballvalves, or precision metering valves might also be used. In the case ofa two position ball valve, the open position would allow completeunconstrained flow. In the closed position, the ball valve has a smallorifice allowing limited flow. Of course, variants from these conceptsare possible to achieve the same results. The plugs, nozzles, filtersand sintered materials can be positioned in the tube between thecompressed seal footprints areas as a method of holding the devices.

Another approach that can be used to prevent preform draining is toregulate the vacuum on the inner bag. Reducing the vacuum level reducesthe flow rates in the tubes. The preform has a lower tendency to drain,especially for more viscous resins that have sufficient body to movethrough a preform as a continuous pool. The resin also has littletendency to separate into discrete fluid bodies. Using this approach,the inner bag vacuum level is typically dropped from 29+ inches Hg to22-27 inches Hg. Shortly after dropping the vacuum level, the bubblingwill stop as with the throttling devices. A problem with this approachis that the inner bag may move toward the outer bag because of thereduced vacuum. Movement decreases preform compaction and ultimatelyproduces lower fiber volume composites.

After the preform is completely filled with resin and the resin flowrate is constant, the feed and vacuum tubes are clamped closedsimultaneously with sheet metal or welding pliers. The vacuum source isdisconnected from the drop out tank. Both vacuum tubes and the feed tubeare cut near the welding pliers. The resin in the vacuum tubes is suckedinto the drop out tank and the resin in the feed tube drains into thefeed container. The process results in complete resin reclamation andallows real time mass balances to be performed. The ends of the cutvacuum tubes and feed tubes are sealed with pressure-sensitive adhesivetape and then wrapped with vacuum bag sealant tape. The tube seals aresimply a redundant measure to prevent air from entering the inner bag inthe event the welding pliers fail to isolate the inner bag fromatmospheric pressure. Prior to beginning cure, the bulk resin in thefeed can and the drop out can is removed from the oven to preventunwanted hazardous exothermic reaction. Likewise, all other tool andconsumable materials are removed prior to closing the oven for cure.

Our preferred process allows resin recycling or recirculation. In somecomplex infusions where, for example, separate wave fronts convergetogether, extra resin may need to be purged preform to remove trappedair or voids. Resin recirculation rather than continuous purgingminimizes resin waste and expense. With recirculation, excess resin istypically charged to the system to have a reasonable working volume. Theresin is allowed to accumulate in the drop out can. Once the resin inthe supply can begins to run low, the feed and vacuum tubes are clampedshut. The vacuum source to the drop out tank is disconnected and thevacuum is released using the quick connect fittings. With the vacuumreleased, the lid on the drop out tank can be removed, and resin drainedinto the can from the tubes. The resin in the drop out can istransferred to the source container. Sufficient time, usually about 5minutes, is given to allow entrained air to percolate out of the resinbefore flow is reinitiated. The drop out tank is reassembled andevacuated with a separate isolated vacuum pump to prevent any possiblevacuum decrease in the outer bag. Once the tank is at the originalvacuum level, all tube clamps are released simultaneously establishingflow again. The process can be repeated until all voids and bubbles areremoved from the preform. At this point, the infusion can be terminated.

Batch infusion and cure processing is possible. The only potentialconstraint is the number of infused parts that can be loaded in theoven. If parts are infused serially, excess working resin from any priorcompleted infusion can be used as is or blended with virgin resin forsubsequent infusions.

As previously described, a vacuum tube mounted on a stiffener, forexample, can be used to infuse that stiffener and thereafter beconverted into a feed line for the next infusion bay or section (FIG.4). This conversion between feed and vacuum uses a T or Y fitting. Oneleg of the T connection is connected to a resin supply, but can beclamped or closed with a valve while the other leg is left open andconnected to the vacuum drop out tank. Once resin begins to fill thevacuum line and clears the T fitting with no bubbles, the tube exitingthe bag is closed and the leg to the resin supply is opened. The supplytube fills with resin and purges air from all tubing. Once the tubes arefilled, the vacuum line leading to the drop out tank is closed shut andthe tube exiting the bag is opened allowing a new resin supply to feedthe preform.

Our process might be usable to produce carbon/carbon and ceramic matrixcomposites through multiple infusions, cures, and densification of thesame preform materials.

Resin Cure

Once the infusion is completed, high vacuum must be maintained in theouter bag of the infused preform throughout the cure cycle, especiallyimmediately before and during resin gelation. Vacuum loss during thiscritical stage will cause the inner bag to relax, increasing the volumein the inner bag. The infused preform will swell. Because resin cannotbe added in the closed system, the swelling reduces the hydrostaticpressure which produces surface porosity and voids, reduced preformcompaction, and lower fiber volumes.

Because of the critical nature of the outer bag vacuum level, we usevacuum transducers, data acquisition equipment, and Labview software tomonitor continuously the pressure throughout the cure. Oven and tooltemperatures are also continuously recorded and displayed in real time.Because of the risk to vacuum integrity, monitoring thermocouples arenot introduced into the bag. Viewing windows in the oven are desirableto observe the cure in progress.

Our process has been developed primarily for thermally curing (heating)the resins. Alternative curing methods such as electron beam curing, UVcuring, and microwave curing can be used with heat or independently orin combination with appropriate resins and bagging materials.

Low temperature curing resins have the advantage that they can be curedon low cost tooling with better dimensional control, particularly oncomplex co-cured assemblies. Some room temperature curing resins havehazardous exothermic reactions in bulk quantities. To circumvent thisproblem, special meter mix, vacuum de-airing equipment is necessary todispense resin into the supply can on demand.

Postcure

Postcure requirements depend on the infusion resin and the desiredoperating temperature of the structure. Postcure may be performed on thebond jig tooling, simple support fixtures, or free standing with orwithout glass fabric covers to protect from oxidation or foreigncontamination. If postcure is desired, generally it subjects thecomposite to a temperature cycle for an extended period.

Trim and Inspection

Typically composites, including those made using our process, must betrimmed around the periphery to the final, desired dimensions for thepart. Trimming can be done with a router, water jet cutter, by roughcutting and grinding to a trim line, or any other appropriate method.

Certain features, such as blade stiffeners, and pi or clevis type jointscan be net molded using “soft” or “hard” binderized preforms or fibrouspreforms without binders, such as multiaxial warp knit fabrics that areinherently stable. Deflashing excess resin is the only operationrequired. Net molded features can be laid up in the net configuration oroversized as a preform. For oversized preforms, the layup can be trimmedbefore infusing using the layup tooling as a trim guide. Soft binderizedpreforms can be cut flush with the tool blocks after a room temperatureor low elevated temperature vacuum bag debulk to provide definition andconsolidation. Semi-rigid “hard” binderized preforms tend to give betteredge definition when trimmed. These semi-rigid preforms are typicallymade using an elevated temperature vacuum bag debulk.

The composites can, then, be inspected using any one or all of manynon-destructive inspection (NDI) techniques of the type typically usedto inspect autoclave-cured composites, including ultrasonic andradiographic techniques. Inspection may be avoided if certain in-processcontrols are used throughout the manufacturing process. Visualobservation of the flow through windowing in the breather material, useof optically clear tooling made, for example, from PYREX or LEXAN, andoptically clear or translucent tubing provide indications of qualityduring the infusion. Similarly, mass-balances, infrared flow frontdetection, imbedded, remotely queried sensors, or flush tool mountsensors can provide in-process indications of quality. Visual inspectionof the laminates after processing generally is a good indicator of theirquality. If the laminates do not have surface porosity (particularly onthe tool side), if the thickness is within nominal limits, and if thecomposite rings when “coin tapped” (see, e.g., U.S. patent applicationSer. No. 08/944,885), the laminates will likely pass ultrasonicinspection. If any surface voids appear on the parts, ultrasonicinspection is warranted. Because we have determined that there is astrong correlation between the existence of surface voids and theoverall composite quality, simple inspection for surface voids cansignificantly reduce or even eliminate more sophisticated inspectionusing expensive ultrasonic, laser, or radiographic processes.

Advanced Processing Techniques

Our process is able to fabricate complex structural assemblies, such asthe I-beam stiffened skin shown in FIGS. 7 and 8. Conformal tubefairings located between I-beams are used to deliver resin in thedifficult to reach bay zones. Edge feed tubes supply resin to the edgesof the skin. Vacuum tubes located at the tops of the I-beams are used todraw the resin through the skin and up through the I-beams. To infusethis assembly, all feed lines 8 a are opened at the same time. The wavefronts generated from the three feed sources converge around the base ofthe I-beams and are drawn up into the I-beam toward the vacuum tubes.When the resin reaches the top of the I-beam web, the flow splits to wetout each flange on the cap. The resin eventually wraps around the caulplate and works its way to the vacuum tube 8 b. The infusion iscompleted, and the composite cured in the normal manner. This techniquecan be extended to produce assemblies with a large number of bays as maybe found in aircraft wings.

Another example of the capability to produce complex structure with ourpreferred process is shown in FIGS. 9 and 10. The intersecting bladestiffened panel uses binderized material for forming the intersectingblades on simple aluminum block tools. Binderized material can be usedif desired, but it is unnecessary for the skin 905. To infuse thepreform only one feed line and one vacuum line are required provided thepreform is vertically infused and novel passive vacuum chambers (PVCs)are used. The part is bagged in the horizontal position. Once bagged,the base plate can be flipped in the vertical orientation without anybagside tool movement. As the preform is infused, the resin fills thevertical blade with no plumbing required. Passive vacuum chambers areused to pull resin into the horizontal blades and provide some limitedpurge capability. Air Dam I gum rubber sealant is used at the ends ofthe blades in conjunction with the flow media to eliminate mark off atthe blade terminations. The intersecting blades are net molded with onlyminimal flash removal required.

FIG. 11 shows the bagging for a multiple J-stiffened panel. Again, thepreform is bagged horizontally and flipped vertically for the infusion.One feed line is used at the bottom of the skin and one vacuum line atthe top of the skin. The two outside J stiffeners use passive vacuumchambers to pull resin into the stiffeners and to provide some limitedpurge capability. The center J stiffener has an active vacuum tube orspring. Once the resin is pulled into the J and purged, the vacuum lineis converted to a feed line using the techniques previously described.This conversion method permits fabrication of very wide assemblies withno mark off from plumbing devices.

We believe that the following concepts of the present invention that wehave described are novel liquid molding techniques of the presentinvention alone or in combination.

-   -   1. Double Bag Infusion    -   2. Binder Technology    -   3. High Elongation Bagging Films    -   4. TEFLON-impregnated Flow Media    -   5. TEFLON Tubing in Infusion Processes    -   6. TEFLON Tubing Etch Process    -   7. Large Tubing Footprints in Seal Locations    -   8. TEFLON Tube Forming for Zero Stress Bends    -   9. Tubing Strain Relief Devices    -   10. Inert Barrier Films for Bag Protection    -   11. Low Flow Gum Rubber Edge Sealant    -   12. Passive Vacuum Chambers (PVCs)    -   13. Conformal Tube Fairings (CTFs)    -   14. Flexible Tooling for Contoured Blade Stiffeners    -   15. External Tube Clamping Feature    -   16. Drop Out Tank and Can for Easy Cleanup and Recirculation    -   17. Positions of Feed Can and Drop Out Cans    -   18. Recirculation Techniques for Difficult Infusions    -   19. Integrally Cast Seal on Drop Out Tank Lids    -   20. Real Time (or Pseudo-Real-Time) Mass Balances to Control        Fiber Volumes    -   21. Infrared Flow Front Detection    -   22. Radio Frequency Identification Embedded Tags.    -   23. PYREX Optical Tooling    -   24. Coarse Flow Media Optical Breather    -   25. Split Swage Tool    -   26. Gum Rubber Sealant at Discontinuities for Reduced Mark off        and Bridging    -   27. Pocket Skeletal Structures    -   28. Heated Vacuum Dry Out Procedures    -   29. Inclined or Vertical Infusion Orientations    -   30. In Mold Binderization    -   31. Design with Feed Capacity Greater than Pull Off Capability    -   32. Vacuum Tube Throttling at End of Infusion    -   33. Feed Tube Resin Constriction at Start of Infusion    -   34. Techniques for Infusing Truss Reinforced Sandwich Panels    -   35. Techniques for Infusing Intersecting blades, hats, pi's,        J's, I's, and C's    -   36. Techniques for Fabricating Door Sill Features    -   37. Leading Edge Sandwich Structures    -   38. Batch Processing Capability    -   39. M-20 Cyanate Ester Catalyzed or Uncatalyzed Binder Solutions    -   40. 5250-4-RTM Binder Solution    -   41. Robotic Spray Application of Binders for Precision Delivery    -   42. Electrostatic Spray Application of Binders for Improved        Transfer Efficiencies    -   43. TOWTAC Binderized Tow Materials    -   44. Automated Concept for Producing and Packing Large Quantities        of Binderized Materials    -   45. Techniques to Produce Soft and Semi-Rigid Preforms    -   46. Production of Tools From Low Temperature Resistant Master        Tools    -   47. Surface Deposition of Binder on Preform Materials    -   48, Binders with Low Degree of Cure for Melt in and Chemical        Bonding with Infusion Resin    -   49. Multiple Infusion Techniques, Especially to Produce        Densified Carbon/Carbon and Ceramic Matrix Composites    -   50. Integration of Specialty Materials such as Ground Planes,        R-Cards, etc.    -   51. Inner and Outer Bag Sealant Placements and Tool Preparation    -   52. High Integrity Seal Features    -   53. Vacuum Forming Conformal Bagging Films    -   54. Resin Reclamation and Blending    -   55. Vacuum Control and Maintenance Prior to Gelation    -   56. Use of Precision Vacuum Transducers for Early Leak Detection    -   57. Use of Wash Out Tooling to Create Stiffened Skins    -   58. Fluid Delivery Through the Bag Seals

The outer bag in the double bag system reduces thermal oxidation of theinner bag. This translates into a stronger bag that is less susceptibleto leakage during cure at high temperatures. The outer bag and breatherbuffer the inner bag from handling damage that can occur in many ways.The outer bag applies pressure to the inner bag seals and improves thesealing effectiveness of those seals. The pressure on the inner bagseals overcomes bag peel stresses that can open up leaks, particularlyat pleated seal locations. Because the outer bag encapsulates the innerbag, seals can not be worked loose in the convective environmenttypically found in ovens. If a leak should occur in the inner bag, theresult is not necessarily catastrophic as it generally is for single baginfusions. A leak in the inner bag will cause resin to flow into theouter bag. Corrective actions are possible with accelerated cures andbleed control techniques. A ruptured bag in a single bag environmentallows air to enter the bag. The bag can swell and porosity can becontinuously introduced into the laminate, resulting in catastrophicfailure. Bag integrity differences between single bag infusiontechniques and double bag techniques may not be significant whenproducing small, simple, low value composites. When attempting toproduce large and/or complex composite assemblies, such as compositewings, the significance of the integrity differences is dramaticallyamplified. It is wise and prudent to use the double bag technique oversingle bags when producing these types of structures. Yield, integrity,and process robustness become far more important factors in reducingoverall cost than eliminating the cost associated with a second bag.

While we have described preferred embodiments, those skilled in the artwill readily recognize alterations, variations, and modifications thatmight be made to the process or the resulting composites withoutdeparting from the inventive concept. Therefore, interpret the claimsliberally with the support of the full range of equivalents known tothose of ordinary skill based upon this description. The examples aregiven to illustrate the invention and not intended to limit it.Accordingly, limit the claims only as necessary in view of the pertinentprior art.

1. A double vacuum chamber resin infusion method for a preformcomprising: locating a preform on a mold; sealingly bagging the preformto the mold with an inner bag forming a first vacuum chamber; sealinglybagging the inner bag to the mold with an outer bag forming a secondvacuum chamber; evacuating the first vacuum chamber; evacuating thesecond vacuum chamber with the pressure in the second vacuum chamberbeing greater than the pressure in the first vacuum chamber; andinfusing a resin into the preform using a vacuum-assisted resin transferapparatus while maintaining the pressure in the first and second vacuumchambers.
 2. A double vacuum chamber resin infusion method for a preformaccording to claim 1 further comprising substantially debulking thepreform by evacuating at least the first vacuum chamber prior toinfusing the resin.
 3. A double vacuum chamber resin infusion method fora preform according to claim 1 further comprising elevating thetemperature of at least the first vacuum chamber and then evacuating atleast the first vacuum chamber.
 4. A double vacuum chamber resininfusion method for a preform according to claim 1 further comprisinglocating passive vacuum chambers within the first vacuum chamber.
 5. Adouble vacuum chamber resin infusion method for a preform according toclaim 1 further comprising asscmbling the preform by tackifying thepreform with a tackifier solution prior to bagging.
 6. A double vacuumchamber resin infusion method for a preform according to claim 1 furthercomprising: locating a flow control media between the inner bag and thepreform; and infusing the resin into the flow control media with theresin passing through the flow control media and then into the preform.7. A double vacuum chamber resin infusion method for a preform accordingto claim 1 wherein the flow control media includes fill that act asweirs to the infusing resin.
 8. A double vacuum chamber resin infusionmethod for a preform according to claim 1 further comprising locating abreather between the inner bag and the outer bag.
 9. A double vacuumchamber resin infusion mcthod for a preform according to claim 6 furthercomprising tilting the preform and the flow control media at an angleoff horizontal and then infusing the resin into the flow control mediawith the resin passing through the flow control media and then into thepreform.
 10. A double vacuum chamber resin infusion method for a preformaccording to claim 9 wherein the tilted flow control media has a lowestpoint and infusing the resin into the flow control media at the lowestpoint.
 11. A double vacuum chamber resin infusion method for a preformaccording to claim 1 further comprising coupling at least one vacuumpump to the first vacuum chamber via at least one first vacuum tube andcoupling at least one vacuum pump to the second vacuum chamber via atleast one aecond vacuum tube.
 12. A double vacuum chamber resin infusionmethod for a preform according to claim 11 further comprising throttlingthe at least one first vacuum tube while infusing a resin such that theinner bag is substantially prevented from relaxing behind a wave frontof resin when resin is infused into the preform.
 13. A double vacuumchamber resin infusion method for a preform according to claim 1wherein: the first vacuum chamber comprising a first space bounded byand including the inner bag and the mold; and the second vacuum chambercomprising a second space bounded by and including the inner bag, themold, and the outer bag.
 14. A double vacuum chamber resin infusionmethod for a preform comprising: locating a preform on a mold; baggingthe preform to the mold with an inner bag forming a first vacuumchamber; bagging the inner bag to the mold with an outer bag forming asecond vacuum chamber; evacuating the first vacuum chamber such that thefirst vacuum chamber collapses substantially against the preform;evacuating the second vacuum chamber with the pressure in the secondvacuum chamber being greater than the pressure in the first vacuumchamber and such that the second vacuum chamber collapses substantiallyagainst the first vacuum chamber; and infusing a resin into the preformusing a vacuum-assisted resin transfer apparatus.
 15. A double vacuumchamber resin infYision method for a preform according to claim 14further comprising substantially debulking the preform by evacuating atleast the first vacuum chamber prior to infusing the resin.
 16. A doublevacuum chamber resin infusion method for a preform according to claim 14further comprising elevating the temperature of a least the first vacuumchamber and then evacuating at least the first vacuum chamber.
 17. Adouble vacuum chamber resin infUsion method for a preform according toclaim 14 further comprising locating passive vacuum chambers within thefirst vacuum chamber.
 18. A double vacuum chamber resin infuision methodfor a preform according to claim 14 further comprising assembling thepreform by tackifying the preform with a tackifler solution prior tobagging.
 19. A double vacuum chamber resin infusion method for a preformaccording to claim 14 further comprising: locating a flow control mediabetween the inner bag and the preform; and infusing the resin into theflow control media with the resin passing through the flow control mediaand then into the preform.
 20. A double vacuum chamber resin infusionmethod for a preform according to claim 19 wherein the flow controlmedia includes fill fibers that act as weirs to the infusing resin. 21.A double vacuum chamber resin infusion method for a preform according toclaim 19 further comprking tilting the preform and the flow controlmedia at an angle off horizontal and then infusing the resin into theflow control media with the resin passing through the flow control mediaand then into the preform.
 22. A double vacuum chamber resin infusionmethod for a preform according to claim 21 wherein the tilted flowcontrol media has a lowest point and infusing the resin into the flowcontrol media at the lowest point.
 23. A double vacuum chamber resininfusion method for a preform according to claim 14 further comprisingcoupling at least one vacuum pump to the first vacuum chamber via atleast one first vacuum tube and coupling at least one vacuum pump to thesecond vacuum chamber via at least one second vacuum tube.
 24. A doublevacuum chamber resin infusion method for a preform Rccording to cinim 23further comprising throttling the at least one first vacuum tube whileinfusing a resin such that the inner bag is substantially prevented fromrelaxing behind a wave front of resin when resin is infused into thepreform.
 25. A double vacuum chamber resin infusion method for a pretormaccording to claim 14 wherein: the first vacuum chamber comprising afirst space bounded by and including the inner bag and the mold; and thesecond vacuum chamber comprising a second space bounded by and includingthe inner bag, the mold, and the outer bag.
 26. A method for infusingwith resin a preform disposed on a mold, the method comprising: locatingthe preform on a mold: forming a redundant double-bag arrangement by:disposing an inner bag over the pertorm; sealing the inner bag to themold to form an inner vacuum chamber defined by the inner bag and themold; disposing an outer bag over the inner bag; and sealing the outerbag to the mold to form an outer vacuum chamber defined by the outerbag, the inner bag, and the mold; evacuating the vacuum chambers suchthat the outer vacuum chamber has a pressure greater than a pressure inthe inner vacuum chamber such that the inner bag is substantiailyprevented from relaxing behind wave front of resin when resin is infusedinto the perform; and infusing resin into the preform whilesubstantially maintaining the pressure in the vacuum chambers.
 27. Themethod of claim 26 wherein the evacuating step further comprisesevacuating the vacuum chambers such that the bags provide a caul effectwith respect to the perform.
 28. The method of claim 26 wherein theforming step further comprises forming the redundant double bagarrangement such that if one of the vacuum chambers fails, the othervacuum chamber substantially maintains vacuum integrity.
 29. The methodof claim 26 wherein tho evacuating stop further comprises evacuating thevacuum chambers such that the outer bag collapses substantially againstthe inner bag and the inner bag collapses substantially against thepreform.
 30. A method for infusing a preform with resin, the methodcomprising: forming redundant vacuum chambers about the perform suchthat: an inner vacuum chamber is received within an outer vacuumchamber; debulking the preform substantially by evacuating the vacuumchambers such that the outer vacuum chamber has a pressure greater thana pressure in the inner vacuum chamber: if one of the vacuum chambersfails, the other vacuum chamber maintains the preform substantiallydebulked by maintaining vacuum integrity; and infusing resin into thepreform while substantially maintaining the pressures in the vacuumchambers.
 31. The method of claim 30 wherein the perform is disposed ona mold, the forming step further comprising forming the redundant vacuumchambers by: sealing the inner bag to the mold to form the inner vacuumchamber defined by the inner bag and the mold; disposing an outer bagover the inner bag; and sealing the outer bag to the mold to form theouter vacuum chamber defined by the outer bag, the inner bag, and themold.
 32. The method of claim 30 wherein the evacuating step furthercomprises evacuating the vacuum chambers such that the bags provide acaul effect with respect to the perform.
 33. The method of claim 30wherein the evacuating step further comprises evacuating the vacuumchambers such that the outeryacuum chamber collapses substantiallyagainst the inner vacuum chamber and such that the inner vacuum chambercollapses substantially against the preform.
 34. A method for infusingwith resin a preform disposed on a mold, the method comprising: locatingthe preform on a mold; forming a redundant double-bag arrangement by:disposing an inner bag over the perform; sealing the inner bag to themold to form an inner vacuum chamber defined by the inner bag and themold; disposing an outer bag over the inner bag; and sealing the outerbag to the mold to form an outer vacuum chamber defined by the outerbag, the inner bag, and the mold; evacuating the vacuum chambers suchthat the outer vacuum chamber has a pressure greater than a pressure inthe inner vacuum chamber and the bags provide a caul effect with respectto the preform; and infusing resin into the preform when the vacuumchambers are evacuated such that the inner bag is substantiallyprevented from relaxing behind a wave front of resin when resin isinfused into the preform.
 35. The method of claim 34 wherein theevacuating ster further comprises evacuating the vacuum chambers suchthat the outer vacuum chamber has a pressure greater than a pressure inthe inner vacuum chamber.
 36. The method of claim 34 wherein the formingstep further comprises forming the redundant double-bag arrangement suchthat if one of the vacuum chambers fails, the other vacuum chambersubstantially maintains vacuum integrity.
 37. The method of claim 34wherein the evacuating step further comprises evacuating the vacuumchambers such that the outer vacuum chamber collapses substantiallyagainst the inner vacuum chamber.